perf(chat): bound MarkdownState residency with windowed eviction

[Tryanks]

Implements the MarkdownState half of #117 (timeline payload paging deliberately deferred — noted in the issue).

• Window: visible turns ±8 build margin; eviction beyond ±24 (3× hysteresis vs the build margin, on top of the list's 4-viewport measurement overdraw) — scrolling can't thrash build/evict.
• Pins: active/streaming turns, the last two composer-adjacent turns, selection participants; no eviction during an active selection drag.
• Stability: eviction removes only md_states entries — the outer ListState height cache survives, so absolute scroll anchors hold to the pixel (asserted turn 40 + 7px in tests). Rebuilds remeasure only contiguous rebuilt ranges. Tail-follow and the palette one-shot turn-target jump (rebuild → scroll → highlight → consume) keep working.
• Measured (240-turn × 3-entry synthetic session): 720 resident states before → 48 at tail, 78 mid-history — red baseline test failed pre-fix, passes post-fix, with a test-accessible resident count as the regression hook.

Gates: fmt / clippy -D warnings / full workspace tests (825 passed) — green locally.

Closes#117

🤖 Generated with Claude Code

[Tryanks]

Implements the MarkdownState half of #117 (timeline payload paging deliberately deferred — noted in the issue).

• Window: visible turns ±8 build margin; eviction beyond ±24 (3× hysteresis vs the build margin, on top of the list's 4-viewport measurement overdraw) — scrolling can't thrash build/evict.
• Pins: active/streaming turns, the last two composer-adjacent turns, selection participants; no eviction during an active selection drag.
• Stability: eviction removes only md_states entries — the outer ListState height cache survives, so absolute scroll anchors hold to the pixel (asserted turn 40 + 7px in tests). Rebuilds remeasure only contiguous rebuilt ranges. Tail-follow and the palette one-shot turn-target jump (rebuild → scroll → highlight → consume) keep working.
• Measured (240-turn × 3-entry synthetic session): 720 resident states before → 48 at tail, 78 mid-history — red baseline test failed pre-fix, passes post-fix, with a test-accessible resident count as the regression hook.

Gates: fmt / clippy -D warnings / full workspace tests (825 passed) — green locally.

Closes#117

🤖 Generated with Claude Code

[Tryanks]

Implements the MarkdownState half of #117 (timeline payload paging deliberately deferred — noted in the issue).

• Window: visible turns ±8 build margin; eviction beyond ±24 (3× hysteresis vs the build margin, on top of the list's 4-viewport measurement overdraw) — scrolling can't thrash build/evict.
• Pins: active/streaming turns, the last two composer-adjacent turns, selection participants; no eviction during an active selection drag.
• Stability: eviction removes only md_states entries — the outer ListState height cache survives, so absolute scroll anchors hold to the pixel (asserted turn 40 + 7px in tests). Rebuilds remeasure only contiguous rebuilt ranges. Tail-follow and the palette one-shot turn-target jump (rebuild → scroll → highlight → consume) keep working.
• Measured (240-turn × 3-entry synthetic session): 720 resident states before → 48 at tail, 78 mid-history — red baseline test failed pre-fix, passes post-fix, with a test-accessible resident count as the regression hook.

Gates: fmt / clippy -D warnings / full workspace tests (825 passed) — green locally.

Closes#117

🤖 Generated with Claude Code

[Tryanks]

Implements the MarkdownState half of #117 (timeline payload paging deliberately deferred — noted in the issue).

• Window: visible turns ±8 build margin; eviction beyond ±24 (3× hysteresis vs the build margin, on top of the list's 4-viewport measurement overdraw) — scrolling can't thrash build/evict.
• Pins: active/streaming turns, the last two composer-adjacent turns, selection participants; no eviction during an active selection drag.
• Stability: eviction removes only md_states entries — the outer ListState height cache survives, so absolute scroll anchors hold to the pixel (asserted turn 40 + 7px in tests). Rebuilds remeasure only contiguous rebuilt ranges. Tail-follow and the palette one-shot turn-target jump (rebuild → scroll → highlight → consume) keep working.
• Measured (240-turn × 3-entry synthetic session): 720 resident states before → 48 at tail, 78 mid-history — red baseline test failed pre-fix, passes post-fix, with a test-accessible resident count as the regression hook.

Gates: fmt / clippy -D warnings / full workspace tests (825 passed) — green locally.

Closes#117

🤖 Generated with Claude Code

[Tryanks]

Implements the MarkdownState half of #117 (timeline payload paging deliberately deferred — noted in the issue).

• Window: visible turns ±8 build margin; eviction beyond ±24 (3× hysteresis vs the build margin, on top of the list's 4-viewport measurement overdraw) — scrolling can't thrash build/evict.
• Pins: active/streaming turns, the last two composer-adjacent turns, selection participants; no eviction during an active selection drag.
• Stability: eviction removes only md_states entries — the outer ListState height cache survives, so absolute scroll anchors hold to the pixel (asserted turn 40 + 7px in tests). Rebuilds remeasure only contiguous rebuilt ranges. Tail-follow and the palette one-shot turn-target jump (rebuild → scroll → highlight → consume) keep working.
• Measured (240-turn × 3-entry synthetic session): 720 resident states before → 48 at tail, 78 mid-history — red baseline test failed pre-fix, passes post-fix, with a test-accessible resident count as the regression hook.

Gates: fmt / clippy -D warnings / full workspace tests (825 passed) — green locally.

Closes#117

🤖 Generated with Claude Code

[Tryanks]

Implements the MarkdownState half of #117 (timeline payload paging deliberately deferred — noted in the issue).

• Window: visible turns ±8 build margin; eviction beyond ±24 (3× hysteresis vs the build margin, on top of the list's 4-viewport measurement overdraw) — scrolling can't thrash build/evict.
• Pins: active/streaming turns, the last two composer-adjacent turns, selection participants; no eviction during an active selection drag.
• Stability: eviction removes only md_states entries — the outer ListState height cache survives, so absolute scroll anchors hold to the pixel (asserted turn 40 + 7px in tests). Rebuilds remeasure only contiguous rebuilt ranges. Tail-follow and the palette one-shot turn-target jump (rebuild → scroll → highlight → consume) keep working.
• Measured (240-turn × 3-entry synthetic session): 720 resident states before → 48 at tail, 78 mid-history — red baseline test failed pre-fix, passes post-fix, with a test-accessible resident count as the regression hook.

Gates: fmt / clippy -D warnings / full workspace tests (825 passed) — green locally.

Closes#117

🤖 Generated with Claude Code

[Tryanks]

Implements the MarkdownState half of #117 (timeline payload paging deliberately deferred — noted in the issue).

• Window: visible turns ±8 build margin; eviction beyond ±24 (3× hysteresis vs the build margin, on top of the list's 4-viewport measurement overdraw) — scrolling can't thrash build/evict.
• Pins: active/streaming turns, the last two composer-adjacent turns, selection participants; no eviction during an active selection drag.
• Stability: eviction removes only md_states entries — the outer ListState height cache survives, so absolute scroll anchors hold to the pixel (asserted turn 40 + 7px in tests). Rebuilds remeasure only contiguous rebuilt ranges. Tail-follow and the palette one-shot turn-target jump (rebuild → scroll → highlight → consume) keep working.
• Measured (240-turn × 3-entry synthetic session): 720 resident states before → 48 at tail, 78 mid-history — red baseline test failed pre-fix, passes post-fix, with a test-accessible resident count as the regression hook.

Gates: fmt / clippy -D warnings / full workspace tests (825 passed) — green locally.

Closes#117

🤖 Generated with Claude Code

[Tryanks]

Implements the MarkdownState half of #117 (timeline payload paging deliberately deferred — noted in the issue).

• Window: visible turns ±8 build margin; eviction beyond ±24 (3× hysteresis vs the build margin, on top of the list's 4-viewport measurement overdraw) — scrolling can't thrash build/evict.
• Pins: active/streaming turns, the last two composer-adjacent turns, selection participants; no eviction during an active selection drag.
• Stability: eviction removes only md_states entries — the outer ListState height cache survives, so absolute scroll anchors hold to the pixel (asserted turn 40 + 7px in tests). Rebuilds remeasure only contiguous rebuilt ranges. Tail-follow and the palette one-shot turn-target jump (rebuild → scroll → highlight → consume) keep working.
• Measured (240-turn × 3-entry synthetic session): 720 resident states before → 48 at tail, 78 mid-history — red baseline test failed pre-fix, passes post-fix, with a test-accessible resident count as the regression hook.

Gates: fmt / clippy -D warnings / full workspace tests (825 passed) — green locally.

Closes#117

🤖 Generated with Claude Code