feat(tui): lock v2 runtime chrome

[echobt]

Summary

Runtime implementation of the Cortex CLI TUI lock v2. Separate from design pack #20. SPEC + txt grids are vendored so scripts/render-tui-lock-v2.sh can recapture the live session; designer PNGs stay on #20.

Do not merge until Designer cli signs off on runtime captures. Invite Designer cli to QA again (Oding will ping).

CAPTURES_FIXED_PASS

Live MockTerminal recapture of the full SPEC §7 set. Each filename is one unique state — no two PNGs share a sha256.

• PR: feat(tui): lock v2 runtime chrome #21
• Regenerate: ./scripts/render-tui-lock-v2.sh (fails if hashes collide or counts are short)
• 120×40: 77 files, 77 unique hashes
• 40×12: 31 files, 31 unique hashes

Previously aliased frames are now distinct:

• session-user-bars ≠ session-thought ≠ session-assistant
• settings-appearance ≠ settings-row-hover (BAR_HOVER on Show timestamps)
• welcome-cortex ≠ composer-empty ≠ footer-shortcuts

Test plan

• cargo fmt --all -- --check
• ./scripts/clippy.sh
• cargo test -p cortex-tui --lib lock_v2 (unique ANSI frames at both sizes)
• ./scripts/render-tui-lock-v2.sh — 77 + 31 unique sha256
• CI cargo test --workspace / cargo audit on this commit

Attestation (required)

I attest that:

• Security reviewed — auth, exec/sandbox, path traversal, network egress, and secret handling in this diff were reviewed. No secrets, tokens, or keyring dumps are in the change.
• Product-facing errors — user-visible failures use Cortex product copy. API-down paths say The coding service is temporarily unavailable. No raw provider, SDK, or transport names.
• TUI verified — every TUI surface touched in this PR was exercised (build + headless snapshot / ratatui-style test). Runtime PNGs recaptured; no interactive TTY run in this environment.
• Tests added — unit tests cover the new logic; TUI changes include a snapshot or buffer assertion. No mocks that report success.
• No secrets — no API keys, WorkOS secrets, R2/AWS credentials, or .env files are included.

Risk

TUI-only. Alternate screen stays default always. agent is a second bin target on the same main.rs. Ctrl+x is the shortcuts overlay (no longer “clear queue”).