transcript diffs, thinking blocks, error hints, and mouse selection

[jbj338033]

Summary

A set of goat-tui UI improvements, each scoped by necessity against the current code:

• Edit diffs render inline in the transcript (+/− colored), reusing the diff already produced in ToolOutcome.summary; also fixes a latent cache bug where item_signature never hashed the summary.
• Thinking blocks materialize as a collapsible ▸ Thought line (Ctrl+T to expand raw reasoning), preserved across /resume via a new TranscriptEntry::Thinking.
• Errors gain a left rail plus a structured Event::Error.hint with an → /command action line, and actionable hints are backfilled for the five previously bare categories.
• Inline · separators audited by role: whitespace where anchored, → for state→action, · kept only for anchorless peer lists.
• Composer Esc-clear now recovers the wiped draft via history (↑).
• Mouse: drag-to-select with highlight, Cmd+C copy (WYSIWYG minus the structural gutter, CJK-aware, clipboard write offloaded), double-click word select, and click-to-zoom for inline images.
• Scroll: a ↓ N below indicator when scrolled up with content below.

Skipped by necessity (rationale in the plan): keybinding customization, custom themes, header/indicator density, transcript virtualization (already viewport-bounded), scrollbar drag.

Verification

• cargo test --workspace — passed (goat-tui 225, select.rs 11 pure-logic tests)
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo fmt --all -- --check — passed
• Interactive paths (drag-select→Cmd+C, image zoom, thinking toggle) need manual cargo run in a real tty; the full-screen TUI is not driven headlessly.