feat(lab): coding_markdown_v2 - the CC parity pack (audit findings wired in)

Per Eli: don't defer known-coming work. One flag (MIKE_CODING_PARITY, v2
arm = v1 env + it), frozen surfaces untouched (lean schema byte-equality
suite still green):

- Serves the Claude-Code-shaped surface: Glob (pattern-matched files on
  the served plane), a single file_path Read (always cat -n, 2000-line
  default, CC's read-only-the-part-you-need cue), Grep with -A/-B and
  the trained files_with_matches default - descriptions carry the CC
  efficiency cues the audit found missing; the anti-native batch-read
  invitation is gone (audit F3/F4/F6 + axis 3).
- Executor parity behind the flag: the lean paths[] Read intercept is
  bypassed; ripgrep-legal patterns the JS u-flag rejects retry without
  unicode strictness (F5); -A/-B honored per side in both grep loops
  (F6); provider-materialized {offset:1, limit:1} minima read the
  default window, not one line (F7); route surface-drift guard expects
  the swapped tool list.
- Exposure metrics (all arms, future runs): kind=candidate segments -
  grep/find hits, inventory opening lines - no longer count as exposure
  (F1/F10, the saturated documents_read); tracked separately as
  documents_candidate_only + candidate_span_chars.

Proof: plane probe md/plain/parity all green incl. files_with_matches
default, -A context rows, u-flag fallback match, minima guard (369
lines); served-surface probe v2 6/6 (sha, flags, CC tool list) with v1
5/5 and treatment 9/9 regressions intact; localToolWiring +
structureIndex suites 39/39; tsc clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pfUVhNFTRvhYGXBwoKNj6