fix(tui): preserve legacy Alt navigation

[yxlyx]

Closes #524. This PR is intentionally stacked on #699 and contains one additional commit; once #699 merges, this PR can be retargeted to main without changing its diff.

What changed

• Decode legacy ESC ESC <CSI> Alt+Left/Right/Up as one navigation event instead of Escape followed by navigation.
• Preserve framing across every read split, bounded delayed delivery, full-buffer recovery, and dropped-head recovery.
• Keep genuine lone/double Escape behavior after the ambiguity window and preserve recognizable bracketed prose instead of guessing it is navigation.
• Add reachable fullscreen-TUI regressions for idle, live turn, background compact/bash/files, overlay, and bracketed-paste trajectories.

Why

Problem / failure mode

Some terminals encode legacy Alt+Arrow as ESC ESC plus CSI. The parser previously emitted the first Escape immediately, so the shared TUI dispatcher could cancel a model turn or background operation, close an overlay, end paste, or arm rewind before processing the arrow. Split and delayed terminal reads made this destructive behavior timing-dependent.

Reason for this approach

The incorrect event originates in the shared input framing path, so the fix keeps the complete legacy sequence ambiguous until it can be decoded once. The bounded stall/recovery state from #699 already tracks live operations and late terminal tails; building on it protects every fullscreen TUI trajectory without adding downstream guards or duplicating #537's broader recovery machinery.

Constraints and trade-offs

The terminal byte stream cannot perfectly distinguish ESC ESC [Ax] as Alt+Up then x] from genuine double Escape followed by literal [Ax]. The implementation uses narrow evidence: exact navigation, split input, and navigation followed by ordinary coalesced text win as navigation; recognizable bracketed prose wins as text; an incomplete sequence beyond the existing ~300 ms idle / ~1 s live window resolves as genuine Escape. This preserves cancellation responsiveness while covering the reported 50–250 ms delivery range.

Rejected alternatives

• Suppressing Escape only in live-turn code was rejected because the same phantom event affects background operations, overlays, paste, rewind, and idle navigation.
• Copying the old TUI audit triage: 16 unverified findings from the out-of-the-box review #524 draft was rejected because it mixed paste normalization, stale modifiers, SS3/orphan recovery, and other TUI escape/death audit round 2: 12 unverified findings #537 concerns.
• Basing directly on main was rejected because the delayed runtime seam needs fix(tui): harden split input and paste recovery #699's operation-aware stall/recovery behavior; restating that work would create overlapping PRs.

Verification

• GRAFF_TUIGUARD_JOBS=1 scripts/eval-tier1.sh — green in 562s.
• Unit suite: 1860 passed + 1 platform skip (1861 total).
• TUI suite: 514/514.
• PTY/tuiguard: 18/18.
• Formatting, 600-line ceiling, test reachability, build, invariants, and SDK drift checks all passed.
• Independent implementation, adversarial-test, and final-review agents exercised split/delayed input, coalesced printable keys, [Alice] prose, unmodified and 1;3 CSI forms, all background operation kinds, overlay movement, paste, full-buffer recovery, and genuine Escape behavior. Final review reported no blockers.

[justrach]

Folded onto release/v0.0.283 (PR #703) as e674794 — this is the complete CSI-only Alt+arrow framing (ESC ESC [ stays one nav event; SS3 ESC ESC OA stays Escape). Replaces the smaller leftover that was already on the cut.

Leaving this PR open for you to close as superseded.

[justrach]

Shipped in v0.0.283 via #703 (e674794). Closing as superseded — not merging this branch to main.