TUI Phase 7 — finish flash, richer markdown, block nav, click-to-fold

[davidrhodus]

Phase 7 — the last four grok-build TUI items, in one phase

Stacked on #102 (tui-phase6). This PR's base is tui-phase6, so the diff shows only Phase 7. Merge #102 first; GitHub will retarget this to main automatically.

Four viewing/affordance features:

7a — Finish flash

When a turn settles, the status line briefly glows in the outcome's color — green done, red failed, amber warning, neutral otherwise — fading back to muted over ~450ms. A new App.finished_at stamp (set on the working true→false transition) plus a pure flash_weight linear-decay helper. Idle ticks already redraw every ~120ms, so the fade animates smoothly.

7d — Richer markdown

• Pipe tables: the |---|:--:| separator becomes a ruled ├─┼─┤ divider; data/header rows get muted │ cell separators.
• Links: [label](url) renders the label underlined in the link color with the URL trailing dimmed; bare http(s)://… runs autolink (trailing sentence punctuation left outside).
• Task lists: - [ ] → ☐, - [x] → ☑ (success-colored).

Rendering stays line-local to fit the streaming line-by-line commit path, so table columns align only as well as the source padded them — noted as a deliberate limitation (cross-row alignment would need buffering that fights the streaming architecture).

7b — Block navigation (Ctrl-B)

A cursor over tool-output blocks: ↑↓/jk move, Enter/Space fold or unfold one block independently of the global Ctrl-O, Esc/Ctrl-B exit. The selected block gets a marker line and the view follows the cursor. New per-block expanded override on the ToolOutput entry (moves with the entry through transcript capping — no fragile index set).

7c — Click-to-fold

A left click folds/unfolds the tool-output block under the pointer, mapped through geometry the render pass caches (inner rect, scroll offset, per-block wrapped-row spans). Mouse capture was already enabled for scroll-wheel support, so this is purely additive — no new text-selection regression.

Tests & gates

• New unit tests: flash_weight decay; markdown links/autolinks, task lists, pipe tables (+ prose-with-a-pipe negative); block-nav key handling with independent per-block folding and cursor clamping; per-block expanded flatten; click→block coordinate mapping (+ out-of-area no-op). 164 hi-tui tests pass.
• cargo fmt --all --check, full clippy gate (-D warnings), and the file-size ratchet all pass (bumped render.rs, app/render.rs, app/commands.rs, lib.rs, tests.rs).

Closes out the grok-build TUI adoption (Phases 1–6 handle theme, gutters, folding, panels, sticky headers, syntax highlighting, and the live accent wave).