feat(#3840): generate docs/FEATURES.md from per-feature fragments (#3845)

* feat(#3840): generate docs/FEATURES.md from per-feature fragments

docs/FEATURES.md was hand-maintained, and every feature PR wrote into two
shared mutable cells: the '### N.' heading whose integer was hand-allocated at
authoring time, and the hand-maintained table of contents. Concurrent PRs all
picked the same next integer, and two PRs adding differently numbered features
still collided on the TOC. #3831 was renumbered 165 -> 166 -> 167 -> 168 across
successive rebases, each collision also costing a full matrix verification run
because the sha-keyed pass marker dies with the rebase.

Mechanism: one fragment per feature at docs/features/<slug>.md carrying
id/title/group (and an optional order) in frontmatter, consolidated by
scripts/gen-features.cjs --write|--check into a marker-delimited region of
docs/FEATURES.md that holds BOTH the TOC and every section body. Group headings
and their order are derived too - a group sorts by its lowest-ordered member -
so there is no shared registry to edit either; optional per-group prose lives in
docs/features/_groups/<slug>.md. A contributor adds exactly one new file.
Wired into regen:derived and lint:generated-sync alongside the eight existing
generators, matching gen-adr-index.cjs's CLI shape and typed-REASON reporting.

Migration froze all 168 existing numbers verbatim: identical section set,
identical order, identical bodies. Two defects found in the tree are fixed
inline rather than carried forward - the '## Related' block had been spliced
into the middle of the document, orphaning §142's Reference line, and four
inbound anchors were already broken on next (FEATURES.md#runtime-identity in
two files, and #143-spec-phase-edge-completeness-probe off by one). Since the
repo has no link checker, --check now validates every inbound
FEATURES.md#anchor by resolved target, so that class cannot ship silently
again; locale FEATURES.md files resolve elsewhere and stay out of scope.

Refs #3840

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* fix(#3840): carry upstream §69 delta into its fragment and harden the generator

Review found section 69 missing '[--strict]' and REQ-STATE-05/06 versus
origin/next. Root cause was a stale base, not extraction loss: those lines
landed in 394bf384b (#3844) AFTER this branch forked at 63abcface, and
'git diff 63abcface origin/next -- docs/FEATURES.md' is exactly that hunk.
Merging origin/next auto-applied the hunk into the GENERATED region, which
--check immediately reported as stale; the delta is now carried in
docs/features/statemd-consistency-gates.md and regenerated from there.

--write is now fail-closed. It previously rendered the region even with
violations outstanding, warning only on stderr and exiting 0, so a
'--write && git commit' chain could commit a FEATURES.md carrying two
colliding sections. It now refuses and exits 1; --force is the explicit
override and says so in the report. The test that pinned the old behavior now
pins the refusal, plus the --force override and its scoping.

Marker forgery is rejected at two layers. A fragment body containing
'<!-- FEATURES:START' or '<!-- FEATURES:END' is a typed
body_forges_region_marker violation (fragments and group notes alike), and
spliceIntoFeatures anchors the end boundary with lastIndexOf instead of
indexOf, so a marker that reaches the document by any other route can only
make the generated region grow, never shrink. Matching is on marker PREFIXES,
so a decorated variant comment cannot slip past.

Symlinked corpus entries are refused with a typed dirent_not_regular_file
rather than read. A fork PR could otherwise commit docs/features/evil.md as a
symlink to any readable path and have the generator inline those bytes into
the committed docs/FEATURES.md on the next regen.

Equivalence re-verified with a method that cannot cancel out. The first
check extracted both operands with the same body-normalising helper, so
anything that helper dropped was dropped on both sides. The replacement runs
two independent passes: a global content-line multiset diff with no
per-section logic at all (0 gained, 19 lost, all 19 the stale hand-written
mini-TOC links this change deliberately deletes), and a per-section
byte-exact body diff carrying a coverage assertion that fails loudly per file
when the extractor accounts for fewer lines than the file contains. That
assertion caught two blind spots in the checker itself. 168/168 sections
present, order identical, one intended body difference (§142 regains the
Reference line orphaned by the misplaced '## Related' block).

Refs #3840

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* chore(#3840): backfill changeset PR number

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

---------

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

docs/how-to/diagnose-a-foreign-gsd-tools.md

@@ -124,7 +124,7 @@ command -v gsd-tools || echo "gsd-tools: NOT FOUND"
 
 ## Related
 
-- [Runtime identity](../FEATURES.md#runtime-identity) — why the launcher resolves `gsd_run`
+- [Runtime identity](../FEATURES.md#168-runtime-identity) — why the launcher resolves `gsd_run`
   rather than asserting identity at every call
 - [`runtime-identity`](../COMMANDS.md#runtime-identity) — the verb's exact output
 - [#3129](https://github.com/open-gsd/gsd-core/issues/3129) — the incident this prevents

docs/how-to/resolve-edge-coverage-findings.md

@@ -4,7 +4,7 @@
 
 **Prerequisites:** A phase whose `/gsd-spec-phase` run has passed the ambiguity gate. The edge-completeness probe (Step 5.5) then runs automatically and presents its findings — you do not invoke it separately.
 
-For the category taxonomy and the reasoning behind front-of-pipeline edge analysis, see [Spec-Phase Edge-Completeness Probe](../FEATURES.md#143-spec-phase-edge-completeness-probe). This guide covers only how to *act* on the findings.
+For the category taxonomy and the reasoning behind front-of-pipeline edge analysis, see [Spec-Phase Edge-Completeness Probe](../FEATURES.md#144-spec-phase-edge-completeness-probe). This guide covers only how to *act* on the findings.
 
 ---
 
@@ -103,7 +103,7 @@ This is the payoff: a resolved edge becomes a unit the goal-backward verifier ac
 
 ## Related
 
-- [Spec-Phase Edge-Completeness Probe](../FEATURES.md#143-spec-phase-edge-completeness-probe) — taxonomy, output schema, and the front-of-pipeline rationale
+- [Spec-Phase Edge-Completeness Probe](../FEATURES.md#144-spec-phase-edge-completeness-probe) — taxonomy, output schema, and the front-of-pipeline rationale
 - [`/gsd-spec-phase`](../COMMANDS.md#gsd-spec-phase) — command reference and flags
 - [Plan a phase](plan-a-phase.md) — where `covered`/`backstop` edges become `must_haves`
 - [docs index](../README.md)