fix(terminal): bound shell history replay

[dignfei]

Summary

• load a bounded 1 MiB tail when selecting Shell sessions, while keeping full scrollback available through the explicit Load full history action
• prevent ordinary Shell wheel/touch scrolling from triggering a multi-megabyte reset and full xterm replay at the top of the bounded buffer
• add server/client timing that separates tmux capture, response preparation, transfer/JSON, and xterm parse time
• avoid duplicate Shell cache replay and fix long-buffer row scanning / replay completion races
• apply tmux history limits correctly on both legacy tmux and tmux 3.7+ without globally trimming unrelated live panes

Measurements

• tmux capture-pane: about 0.03s at 10k lines, 0.15-0.18s at 50k, and 0.31-0.36s at 100k
• Chromium + project xterm replay: 1 MiB ~117ms parse, 8 MiB ~368ms, 32 MiB ~698ms; next paint opportunity averaged ~135ms, ~392ms, and ~717ms respectively

[TERMINAL-PERF] and Server-Timing now expose the same stages on real installations. The xterm metric ends at parser completion; it is not a direct GPU/compositor measurement.

Validation

• targeted terminal/tmux tests for the original bounded-history change
• frontend syntax and diff checks for the explicit Shell scroll guard
• tsc --noEmit
• ESLint, public-asset, formatting, and diff checks
• production build

[Ark0N]

Thanks a lot for this one! Went through it in detail and the split is exactly right: shell selection starts from the bounded 1 MiB tail, full history stays behind the explicit button, and TUI sessions keep their one canonical full replay per page. The tmux version gating is a subtle problem handled well (global default set in the same command queue right before new-session on legacy tmux, per-pane targeting on 3.7+ so unrelated live panes never get resized or trimmed). Nice catch on scrollToLastNonEmptyLine double-counting baseY too, and the parse marker that releases the live-output gate while keeping later output ordered behind the snapshot is a clean solution.

Verified locally on top of master together with #327: full test gate green (5510 tests), typecheck/lint/format/frontend-syntax/public-assets, production build. The Server-Timing + [TERMINAL-PERF] stages will make the next scrollback investigation much less guesswork.

Merging, this ships in 1.20.1.