chore(#2143): fail-loud Result + per-surface write-set contract — Phase 3 (#2251)

* chore(#2143): fail-loud Result + per-surface write-set contract — Phase 3

Phase 3 of epic #2143 (ADR-2143 §5/§6). The three target bugs (#2140, #2112,
#2118) were already fixed tactically on next; this introduces the reusable
structural contracts and rewires the primary #2140 site onto them.

- src/write-set.cts (new): the parse `Result<T> = {ok,value|reason}` (§5) and the
  per-surface write-set (`WriteOutcome {surface, applied, requirement?}`,
  `WriteSet`, `writeSetComplete`) (§6). markdown-table.cts now imports + re-exports
  `Result` from here (single source; distinct from command-routing-hub's Result).
- requirements mark-complete (src/milestone.cts): returns a PER-REQUIREMENT,
  per-surface write-set; `write_set_complete` is true only if every surface of
  every requirement applied — structurally forbidding the #2140 OR-into-one-flag
  masking, including across a multi-ID batch (adversarial-review regression).
  Pre-existing output fields unchanged (behaviour-preserving; #2140 already fixed).
- deriveProgressFromRoadmap (src/phase-lifecycle.cts): removed the vestigial
  null-swallowing try/catch (findTableWithColumns never throws) — ADR §5 no-swallow;
  RoadmapProgress return contract unchanged.
- commit --files (#2112) and milestone complete --dry-run (#2118) left as-is
  (single-surface commit / pre-mutation preview — not genuine multi-surface writes).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#2244): backfill changeset PR number (#2251)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

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

src/markdown-table.cts

@@ -3,17 +3,19 @@
  * (ADR-2143, epic #2143). Pure functions, Node built-ins only, string-in/value-out,
  * no I/O. Compiled by tsc to gsd-core/bin/lib/markdown-table.cjs.
  *
- * NOTE: the `Result<T>` here is the ADR-2143 parse-result shape {ok,value|reason} —
- * deliberately distinct from command-routing-hub's dispatch `Result` {ok,data|kind};
- * the two never mix (different modules).
+ * NOTE: the `Result<T>` here is the ADR-2143 §5 parse-result shape {ok,value|reason},
+ * now defined once in `./write-set.cjs` (the shared fail-loud + write-set seam) and
+ * re-exported here so existing importers of `Result` from this module keep working
+ * unchanged — deliberately distinct from command-routing-hub's dispatch `Result`
+ * {ok,data|kind}; the two never mix (different modules).
  */
 
 import { collectSection, replaceSection } from './markdown-sectionizer.cjs';
+import type { Result } from './write-set.cjs';
+export type { Result } from './write-set.cjs';
 
 // ─── Types ────────────────────────────────────────────────────────────────────
 
-export type Result<T> = { ok: true; value: T } | { ok: false; reason: string };
-
 /** A parsed GFM pipe table: header column names + rows addressed by column name. */
 export interface MarkdownTable {
   columns: string[];