feat(tui): Phase 2B — OpenTUI renderer + --tui CLI flag

[Exelord]

Summary

Ships the minimum-viable interactive TUI promised by Phase 1's design
doc §8. Built on top of Phase 2A's pure-function foundation — zero
reducer/state/selector changes. Single screen for now (multi-view +
overlays land in Phase 3).

Tests: 495 → 497. Manual --tui e2e verified.

Components

File	Purpose
src/tui/tui-shim.ts	Single import site for @opentui/react — replacing the renderer = swap this one file.
src/tui/components/Header.tsx	Run id, status counts, parallel-% gauge (color thresholds per design doc), remote-cache indicator.
src/tui/components/TaskList.tsx	One row per task: status glyph + id + elapsed. Selected row highlighted.
src/tui/components/LogPane.tsx	Buffered stdout/stderr for the selected/pinned task.
src/tui/components/ProgressBar.tsx	▇▇▇░░ N/M X% parallel strip.
src/tui/components/StatusBar.tsx	Bottom keymap hint.
src/tui/App.tsx	Composes them; keyboard handler maps j/k, ?, esc, q/Ctrl-C.
src/tui/tui.ts	Entry. Creates the renderer, wraps the Phase 2A reducer + Observer adapter, runs a 33ms paint debouncer + 1 Hz sparkline sampler.

CLI

• --tui / --no-tui parsed in src/cli/run.ts.
• shouldUseTui() decides whether to mount. Falls back silently
  to the framed-block path when disqualified.
• Explicit --tui prints vx: TUI unavailable (<reason>) when
  blocked. Reason strings exactly match the test matrix from Phase 2A.
• Lazy import of src/tui/tui.ts. Non-TUI runs pay zero
  cold-start cost from @opentui/* (~30–60 ms saved per invocation).

Compile-gate (design doc §2)

OpenTUI mounts cleanly under Bun directly — no compile step needed
for the dev/run path. The single-binary bun build --compile story
is deferred until we actually ship a compiled vx; we'll then follow
path (1) sibling-file install from the design doc.

Tooling

• tsconfig.json gains jsx: react-jsx + jsxImportSource: @opentui/react.
• react@19 + @types/react@19 added as deps (peer of @opentui/react).
• All components use React.ReactNode returns (OpenTUI's intrinsics
  return ReactNode, not ReactElement).

Manual e2e

# Inside a project (TTY): TUI mounts (verified locally; CI env can't).
$ vx run build --tui

# CI/non-TTY: silently falls back to framed-block.
$ vx run build --tui   # in this shell (stdin not a TTY)
# → normal framed output

# Disqualifier surfaces under explicit flag:
$ CI=1 vx run build --tui
vx: TUI unavailable (stdout is not a TTY)
# → continues with framed output

Test plan

• bun src/bin.ts run format — clean
• bun src/bin.ts run lint — 0 warnings, 0 errors
• bun src/bin.ts run test — 497 pass, 0 fail
• New: tests/tui-smoke.test.ts (mount + dispatch + dispose),
  tests/cli.test.ts --tui parser cases
• Manual e2e for --tui, --no-tui, and CI=1 --tui
  disqualifier path

What's NOT in this PR (deferred to Phase 3)

• Multi-view (1-5 keys for Graph / Workers / Bottlenecks / Queue).
• Task Detail + Help overlays.
• Stats sparklines + Cache/RemoteCache + Timeline panels.
• Filter input (/).
• Auto-promote default-on (still opt-in for one release cycle).
• Scheduler cancellation hook (Ctrl-C currently relies on the
  process-level SIGINT trigger).

https://claude.ai/code/session_016HXj6HW6bxSn8EYuKcxTD9

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