feat(tui): follow the viewport with the pinned prompt header and jump on click

[SparkofSpike]

Summary

Follow-up to the pinned user-prompt header: it now tracks the turn the
viewport starts on instead of only the newest message, and clicking it
returns the viewport to the message it names.

1. Per-turn hand-over. The header used to belong to the newest user
   message alone: scrolling up past that message dropped the header
   entirely, and it only came back once the viewport cleared the whole
   turn. It now belongs to whichever user message owns the content at the
   viewport top — the newest user message whose first rendered line sits
   above the viewport. The instant a newer prompt's first line reaches the
   top row, the header hands over to the previous turn's prompt in the same
   frame, with no blank window while scrolling across a turn boundary.
2. Click to jump. The frame that paints the header records its rect and
   the message it names. A left click pins the transcript viewport to that
   message's first line (dropping pending wheel deltas and leaving the live
   tail); clicking outside the header row is left to the transcript
   selection path. The header registers as a hover target with a new
   translated "Jump to this message" label.

Changes

• crates/tui/src/tui/widgets/mod.rs: scrolled_user_prompt_pin scans
  rendered lines newest-first for the first non-blank user-message start
  above top and returns the header line plus the owning message's
  original history index; the caller re-resolves the header against the
  final, row-reserved viewport.
• crates/tui/src/tui/app.rs: ViewportState gains pinned_prompt_area /
  pinned_prompt_message; App::scroll_to_transcript_line mirrors the
  wheel/scrollbar jump path, and pinned_prompt_target_line resolves the
  recorded message against the click frame's layout.
• crates/tui/src/tui/mouse_ui.rs: a left click inside the header rect
  jumps; clicks outside fall through to transcript selection.
• crates/tui/src/tui/ui/frame.rs: hover registration so the header lights
  up under the pointer like every other clickable chrome; the agent-focus
  sampling branch drops the main session's header box.
• crates/localization: new PinnedPromptJumpToMessage message id plus one
  string across all 15 shipped locale packs.

Review follow-up

Two independent reviews were posted on the fork PR (SparkofSpike#5), both
closing as mergeable; every finding was addressed before merge.

• SpikeBot 003 found that a prompt opening on a blank line was picked as
  the pin target and then failed the first-line render check, dropping the
  header instead of handing over to an older message — prompts without a
  renderable first line are now skipped during the scan, with a regression
  test. It also asked for multi-line and non-identity collapsed_cell_map
  coverage, which was added, and confirmed that the header's read
  resolution against the click frame's layout is intentional.
• SpikeBot 005 found that the agent-focus sampling path recorded the
  pinned header's hit box even though the focus pane paints no header (the
  focus banner row became an invisible click/hover target — now dropped),
  and that reserving the header row moved top down on the tail so a
  prompt whose first line was exactly the full-height top row could be
  hidden behind a header naming an older prompt (the header is now
  re-resolved against the final viewport, with a regression test). It also
  asked for the header to key off a message identity rather than a
  frame-bound line offset, and for an end-to-end render→click→land test —
  both are in.
• SpikeBot 003 re-checked the follow-up commits and closed mergeable,
  verifying each fix point by point.

Base and CI note

• This branch has merged upstream main (ad4325063), so it sits on the
  current tree — that commit is also the rustfmt fix for
  crates/tui/src/child_env.rs, which makes cargo fmt --all -- --check
  clean repository-wide.
• The CI Lint job's clippy step may still fail on three findings that
  live on main and are untouched here: child_env.rs:718
  (chunks_exact constant chunk size), mcp.rs:290 and lib.rs:13797
  (needless_return). Every file this PR touches is clippy-clean under
  the CI allow list.

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• cargo fmt --all -- --check — clean repository-wide
• cargo check -p codewhale-tui --all-targets — exit 0
• cargo clippy -p codewhale-tui --all-targets --locked -- -D warnings -A clippy::uninlined_format_args -A clippy::too_many_arguments -A clippy::unnecessary_map_or — no findings in any file this change touches (the three remaining errors are the pre-existing ones noted above)
• cargo test -p codewhale-tui --lib -- pin_helper → 7 passed; 0 failed (hand-over timing, blank-led skips, reserved-viewport repin, filtered-map resolution)
• cargo test -p codewhale-tui --lib -- pinned_prompt → 6 passed; 0 failed (header layout & tail reservation, hit-box recording, click jump, click outside the row, hover registration, render→click→land)
• cargo test -p codewhale-tui --lib -- hover_target → 2 passed; 0 failed
• cargo test -p codewhale-localization → 52 passed; 0 failed (key-parity gates across all packs, including the new message id)
• Verified TUI behavior manually if UI changes — the click-to-jump
  path was verified live; the hand-over timing was iterated on live
  session feedback and is pinned by the unit tests above.

Checklist

• Updated docs or comments as needed (module docs describe the
  hand-over semantics and the click path)
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes
• Harvested/co-authored credit uses a GitHub numeric noreply address
  (no external contribution in this change)

Related Issues

No-Issue: interaction follow-up to the pinned prompt header; no upstream
issue was opened for this.

Attribution

🤖 Generated by SpikeBot 000(CodeWhale-LOCAL)