emrg: TUI multi-line message fix — composer per-line render + preserve line breaks in user markdown (rant 2026-08-19T14:25:55)

[argszero]

Summary

Host rant 2026-08-19T14:25:55 — pasted multi-line text in the TUI lost its line breaks in two places:

1. Composer input box — multi-line text rendered as one auto-wrapped blob
2. Sent user message — Rich Markdown collapsed single \n into spaces → one long line, with the > prefix only on the first visual line

Changes

emrg/client/python_tui/widgets/composer.py — render one Line per logical line:

• The text is split on \n; the first line carries the > prompt, continuation lines a same-width indent (matching ChatRow.render)
• The cursor is drawn on the line it currently sits in (was previously a single flat Line, and buffer.py skips \n characters — the root cause of the flattened input)

emrg/client/python_tui/widgets/markdown.py — UserMarkdown preserves single newlines:

• New _preserve_line_breaks() turns single \n into CommonMark hard breaks (line + two trailing spaces) so RichMarkdown renders each logical line separately
• Blank lines (paragraph separators) and fenced code-block interiors are left untouched (trailing whitespace is significant there)
• The > prefix stays on the first line only; continuation lines keep the same-width indent

Tests

• New tests/test_composer.py (6 tests): multi-line render = one Line per logical line, no prompt on continuation lines, cursor placement on first/continuation lines, empty placeholder
• tests/test_user_markdown.py (+3): single newlines preserved, blank lines remain paragraph separators, fenced code blocks untouched by the hard-break preprocessing
• Suite: 988 collected, 987 passed + 1 skipped; import + CLI checks green (Agent.md count already synced)

[argszero]

✅ LGTM — cycle. CI test + test-windows PASS (run 32227017880); local suite 988 collected (987 passed + 1 skipped); composer per-line render + UserMarkdown hard-break preservation verified.

[argszero]

✅ LGTM — cycle. Second consecutive approval (cycles 912/913); head unchanged, CI test + test-windows PASS, MERGEABLE/CLEAN.

[argszero]

✅ LGTM — cycle. Head unchanged (b16c91f), CI test + test-windows both PASS (run 32227017880). Third consecutive approval.

[argszero]

Maintainer conflict-resolution push (Committer): master advanced e401246 (#869 scheduler removal) while this PR was open; Agent.md doc-count line conflicted (PR said 988 against the pre-#869 base of 979; #869 dropped the suite to 968). Resolved by merging master in and setting the count to 977 (968 + 9 new tests from this PR) — verified locally: 976 passed + 1 skipped = 977 collected. New head c289e09; CI re-running.