docs(#3215): add ADR-3212 lexical seam consolidation (Phase 0) (#3221)

Design-lock ADR for epic #3212 — the lexical layer beneath the #1372
markdown-sectionizer and #2143 table/mutation seams.

Locks seven decisions Phases 1-4 execute against:
- src/pattern.cts as sole owner of dynamic regex construction,
  delegating to the built-in RegExp.escape; the ten private
  escapeRegex/escapeRegExp/escapeRe copies are deleted, not merged
- engines.node floor raised to the Active LTS line (>=24), which is
  what makes RegExp.escape reachable; delegating-shim alternative
  recorded and rejected
- src/text-lines.cts as sole owner of line-terminator handling —
  the primitive the existing no-crlf-fragile-split prohibition lacks;
  brings frontmatter.cts in from the #1372 exclusion
- tokenizer-first for stateful grammars, with a decidable five-condition
  test, generalizing the proven hooks/lib/git-cmd.js token-walk (#3129)
- bounded quantifiers over caller-supplied content
- extend-never-mutate (inherited from ADR-2143 §2)
- prohibition with teeth: no-adhoc-regex-escape, no-unbounded-quantifier,
  no-crlf-fragile-split widened to src/, plus a parity assertion

Explicit non-goal: no wholesale regex-to-parser rewrite. A census found
2,113 regex literals across 317 files; most are correct and stay.

Docs-only. No production code.

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

docs/adr/README.md

@@ -120,7 +120,7 @@ This replaces a hand-maintained table that had drifted to **40 of 65 ADRs** —
 
 <!-- ADR-INDEX:START — generated by scripts/gen-adr-index.cjs; do not edit by hand -->
 
-### Active decisions (56)
+### Active decisions (57)
 
 These govern the system as it stands. Cite these.
 
@@ -181,6 +181,7 @@ These govern the system as it stands. Cite these.
 | [ADR-2782](2782-reviewer-lane-capability-surface.md) | Reviewer Lane — the cross-AI reviewer handoff becomes a declared capability surface | Accepted | — |
 | [ADR-2966](2966-loop-qa-walk.md) | Test the five-step loop as a continuous walk, not isolated points | Accepted | — |
 | [ADR-3180](3180-planning-semantic-model-single-owner.md) | Planning Semantic Model — Single Owner per Derivation | Accepted | — |
+| [ADR-3212](3212-lexical-seam-consolidation.md) | The Lexical Seam — Safe Pattern Construction, Line-Terminator Normalization, and Tokenizer-First Stateful Grammars | Accepted | — |
 | [ADR-3660](3660-runtime-artifact-layout-module.md) | Runtime Artifact Layout Module owns per-runtime artifact placement | Accepted | [ADR-1239](1239-gsd-embeddable-orchestration-engine.md) |
 
 ### Proposed (9)
@@ -214,7 +215,7 @@ Historical record. **Do not follow these** — each names what replaced it, or w
 | [ADR-2264](2264-golden-parity-redesign.md) | Redesign golden-install-parity — single-source manifest builder + split invariant | Superseded | [ADR-2719](2719-emitted-artifact-attribution.md) |
 | [ADR-3524](3524-cjs-sdk-hard-seam.md) | CJS↔SDK hard seam — one source of truth per Shared Module | Superseded | [ADR-0174](0174-retire-gsd-sdk-package-boundary.md) |
 
-_73 ADRs. Generated by `scripts/gen-adr-index.cjs` — run `--write` after adding or restatusing an ADR._
+_74 ADRs. Generated by `scripts/gen-adr-index.cjs` — run `--write` after adding or restatusing an ADR._
 
 <!-- ADR-INDEX:END -->