refactor(#870): extract ROADMAP.md parsing into roadmap-parser.cts (#876)

ADR-857 rollout phase 2b. Move the 6 ROADMAP.md-parsing functions
(stripShippedMilestones, extractCurrentMilestone, replaceInCurrentMilestone,
getRoadmapPhaseInternal, getMilestoneInfo, getMilestonePhaseFilter) + their
interfaces out of core.cts into a new leaf module src/roadmap-parser.cts.
core.cts re-exports them (behavior-preserving); ~9 external callers unchanged.

Resolves the parse/write straddle: roadmap.cts (ROADMAP.md mutation) now imports
its 3 parsing helpers from roadmap-parser.cjs directly instead of reaching
through core. roadmap-parser depends only on leaves (phase-id, planning-workspace,
shell-command-projection) — cycle-free, enabled by phase 2a.

New-CLI-module checklist done (.gitignore, eslint, INVENTORY 92->93 + row,
manifest, ARCHITECTURE, CONTEXT.md "Roadmap Parser Module"). Adds
tests/roadmap-parser.test.cjs (46 tests: behavioral + shim-identity +
adversarial ROADMAP.md fixtures).

Adversarial review surfaced a pre-existing fence-blindness bug in
getMilestonePhaseFilter (matches phase headings inside fenced code blocks);
filed as #875 and left for a separate fix (out of scope for this
behavior-preserving extraction). The two fenced-fixture tests characterize the
current behavior with a #875 reference and flip when it's fixed.

Gates: lint, code-review, security-review, codex adversarial-review (0
correctness findings). gsd-test: clean-build docker + Mac green; the full-suite
docker run's "X is not a function" errors on re-exported symbols were local
incremental-tsc staleness (verified: clean rebuild of the affected files = 195
pass, 0 fail; Mac = 4001 pass).

Closes #870

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -345,6 +345,7 @@ Node.js CLI utility (`gsd-tools.cjs`) with domain modules split across `gsd-core
 | `core.cjs`             | Shared utilities; compatibility re-exports for planning, I/O (`io.cjs`), and phase-id helpers       |
 | `io.cjs`               | CLI I/O primitives — output/error emission, JSON-error mode, large-payload temp-file spillover     |
 | `phase-id.cjs`         | Pure phase-id parsing/matching helpers — normalize, token match, regex builders (extracted from `core.cjs`, ADR-857) |
+| `roadmap-parser.cjs`   | ROADMAP.md parsing — milestone slicing, current-milestone extraction, phase/milestone lookups, milestone-phase filter (extracted from `core.cjs`, ADR-857) |
 | `planning-workspace.cjs` | Planning seam (`planningDir`, `planningPaths`, active workstream routing, `.planning/.lock`)      |
 | `state.cjs`            | STATE.md parsing, updating, progression, metrics                                                    |
 | `phase.cjs`            | Phase directory operations, decimal numbering, plan indexing                                        |

docs/INVENTORY-MANIFEST.json

@@ -324,6 +324,7 @@
       "research-store.cjs",
       "review-reviewer-selection.cjs",
       "roadmap-command-router.cjs",
+      "roadmap-parser.cjs",
       "roadmap-upgrade.cjs",
       "roadmap.cjs",
       "runtime-artifact-layout.cjs",

docs/INVENTORY.md

@@ -370,7 +370,7 @@ The `gsd-planner` agent is decomposed into a core agent plus reference modules t
 
 ---
 
-## CLI Modules (92 shipped)
+## CLI Modules (93 shipped)
 
 Full listing: `gsd-core/bin/lib/*.cjs`.
 
@@ -435,6 +435,7 @@ Full listing: `gsd-core/bin/lib/*.cjs`.
 | `research-store.cjs` | Content-addressed research cache: sha256 keys, per-source TTL staleness, two-tier (user ~/.gsd / project .planning) store |
 | `review-reviewer-selection.cjs` | Reviewer selection/normalization helpers for `/gsd-review` default reviewer policy and precedence |
 | `roadmap-command-router.cjs` | Thin CJS subcommand router adapter for `gsd-tools roadmap` |
+| `roadmap-parser.cjs` | ROADMAP.md parsing — milestone slicing, current-milestone extraction, phase/milestone lookups, milestone-phase filter (extracted from `core.cjs`, ADR-857) |
 | `roadmap-upgrade.cjs` | Migration tool for converting legacy `Phase N` entries to milestone-prefixed `Phase M-NN` convention; `computeMigrationPlan` + `applyMigration` with dry-run default and atomic rollback |
 | `roadmap.cjs` | ROADMAP.md parsing, phase extraction, plan progress |
 | `runtime-artifact-layout.cjs` | Runtime artifact layout module — resolves the artifact directory shapes (commands, agents, skills) for each supported runtime; single source of truth for per-runtime artifact placement (#3663) |