[CTX-0073] chore(test): implement live-socket fixture boundaries (#123)

[Xuepoo]

Summary

Implements the accepted live-socket fixture-boundary design (child of #72)
for bitty-devtools: hoists the in-memory fake into a shared helper and
migrates ipc-socket + client tests to the declared boundary.

Task / Issue

• Priority: P2
• Area: devtools
• Labels: chore, P2, area:devtools
• Milestone: v0.1.0
• Design: CTX-0069 proposal recorded as PX-0319
• Task: CTX-0073
• Review: independent APPROVE recorded as PX-0341
  (reviewer ctx-0073-reviewer-musespark)
• Hash dispute: RESOLVED by commander note PX-0355 (prompt-side
  mislabel; working-tree blobs below ARE the reviewed content; delivery
  of the current tree authorized, no re-review)

Closes #123

Scope

Exactly 3 paths (committed blobs verified at HEAD):

File	Blob
tests/helpers/fake-live-socket.ts	a753c01c
tests/ipc-socket.test.ts	c79ec31a
tests/client.test.ts	42039710

src/ and tests/cli.test.ts untouched (CLI loopback migration is a
follow-up per task description; owned by CTX-0071/0072 lane).

Behavior

• tests/helpers/fake-live-socket.ts (209 lines, no comments): in-memory
  fake at /memory/bitty/fixture.sock (never filesystem) plus
  createScratchLoopback helper (runtime-dir socket, mkdir 0700 /
  chmod 0600, real-uid check, explicit timeoutMs with 1..5000
  ceiling, assertScratchSocketPath absolute + Windows-refused +
  run-owned guards).
• tests/ipc-socket.test.ts: framing suites unchanged logic; live
  round-trip over scratch loopback (1000 ms, try/finally); symlink
  attest + dial both fail with zero Bun.connect dispatches.
• tests/client.test.ts: headless IpcTransport stub unchanged
  synthetic path; live loopback over scratch (1000 ms, try/finally);
  failed-connect adds transportOutgoingLen null zero-dispatch assert.
• Hermetic CI rule preserved: scratch sockets only (host-correct
  absolute; relative/Windows-on-POSIX refused), per-step timeouts
  (fake 100 ms, dial/response 5000 ceiling), run-owned cleanup in
  finally, no production sockets in the unit gate,
  preflight-attest-dial ordering with zero dispatch on failure.

Evidence (hermetic)

• bun test tests/ipc-socket.test.ts tests/client.test.ts: 82 pass /
  0 fail, 380 expects.
• Full gate per PX-0334: bun test full 363 pass / 2986 expects.
• tsc -p tsconfig.json --noEmit: exit 0; tsc build exit 0.
• Prettier 3.9.6 check: clean on all 3 paths.
• oxlint: exit 0 (only pre-existing inspection.ts unused warning
  outside this diff).
• git diff --check: clean.
• git status: only M tests/client.test.ts,
  M tests/ipc-socket.test.ts, A tests/helpers/fake-live-socket.ts.
• No comments / deps / installs / bunx for this change.
• gitleaks pre-push: only 2 findings on synthetic redaction fixture
  token=msk-live-... in tests/tracing.test.ts:651 (CTX-0071 content
  already on origin/main, asserted redacted); our 3-path diff clean,
  no new findings.
• Lefthook pre-commit + commitlint commit-msg hooks pass (no bypass).
• Commit: c5556fb on ctx-0073/fixture-boundary (base 07ebe8d).

Docs sync

No public behavior, protocol, or user-facing diagnostics change;
test-only fixture boundary. No bitty-docs sync required.

Commit hook

Lefthook pre-commit + commitlint commit-msg hooks pass (no bypass, no
bunx, no installs).