enhance(#3618): fold fallow-runner onto the canonical binary resolver (epic #3411 Phase 2) (#3633)

* chore(#3618): fold fallow-runner onto the canonical binary resolver

Epic #3411 Phase 2. src/fallow-runner.cts was the fourth divergent
implementation of Windows binary resolution the epic enumerated —
candidateNames, isExecutableFile, findInPath, findInNodeModules, 40 lines.
All four are deleted; resolveFallowBinary is one seam call.

Two OPT-IN options were added to resolveExecutableBinary to make the fold
behavior-preserving, both defaulting off so Phase 1's callers are byte-identical:

  prependPaths      dirs searched before env.PATH, in order, through the
                    identical per-directory candidate logic. This expresses
                    node_modules/.bin-first precedence without env surgery —
                    the rejected alternative re-introduced the
                    spread-loses-the-proxy hazard the Windows lane caught in
                    Phase 1, at every future call site instead of once.
  requireExecutable POSIX-only accessSync(X_OK); a no-op on win32 where mode
                    bits do not mean execute. Opt-in rather than default
                    because unconditional X_OK breaks #3445's suite, which
                    stages candidates with plain writeFileSync and never sets
                    an exec bit — the repo bans chmod in tests — so every one
                    would resolve to null on POSIX.

Deliberate behavior change on Windows: fallow's prior candidate list ended in a
BARE fallow. The seam never tries a bare name there, so an extensionless file
beside fallow.cmd is no longer resolved. That is the fix, not a regression — the
extensionless file is npm's POSIX sh shim, which CreateProcess cannot run
(#3275). Rows 7 and 8 of the design record it.

Defect found while working, fixed inline: the resolution order was documented
BACKWARDS as PATH-then-.bin in structural-pre-pass.md, docs/INVENTORY.md and
four INVENTORY translations. The code has always been .bin first, and .bin first
is correct — a project-local tool should beat a global one. The archived
changeset is left alone as a historical record.

fallow-runner had no test file at all. tests/fallow-runner.test.cjs is new
(F1-F15) and the seam options are pinned by S1-S12 folded into the existing
dispatch suite. RED proven by execution: with both source files stashed and
build:lib re-run, 7 of 27 probe cases failed.

Refs #3411

* chore(#3618): backfill changeset pr number 3633

* fix(#3618): assert both platform contracts in F4 instead of a POSIX-only premise

Windows CI on #3633 failed F4. The test monkeypatched accessSync to throw and
asserted resolveFallowBinary returned null — but that premise, that the X_OK
check is consulted at all, is POSIX-only by design. requireExecutable is a
deliberate no-op on win32 because Windows mode bits do not mean execute, so the
staged fixture correctly resolved there.

40-design.md's negative-space section already states this carve-out verbatim.
The test contradicted the design it was written from: fixtures were made
platform-adaptive in the previous commit, and this assertion was left
platform-blind.

F4 now asserts BOTH contracts — null on POSIX, resolves on win32 — rather than
skipping either. A t.skip on one lane would have been green and would have left
the win32 carve-out unpinned by fallow's own entry point.

Audited every other row for the same class. F1-F3, F5, F6, F11-F15 hold on both
platforms; F7-F10 and S1-S12 inject platform explicitly and are unaffected. F4
was the only row with a single-platform premise.

The local probe runs on one platform and structurally cannot catch this, which
is why it was green — that limitation is now stated at the top of the probe so a
green probe is not mistaken for platform coverage. The win32 branch was proven
by injecting platform:'win32' with accessSync throwing and asserting it still
resolves.

Refs #3411

---------

Co-authored-by: sim <sim@local>

docs/INVENTORY.md

@@ -499,7 +499,7 @@ Full listing: `gsd-core/bin/lib/*.cjs`.
 | `eval.cjs` | Deterministic eval scoring (compiled from `src/eval.cts`, gitignored) — `computeEvalScore` (coverage*0.6 + infra*0.4, bands 80/60/40) + `cmdEvalScore` CLI domain guard; moves the gsd-eval-auditor's weighted arithmetic out of the prompt into code (#10 / #1579) |
 | `estimate-cli.cjs` | I/O seam over `phase-estimation.cjs` — the `estimate-check` and `estimate-calibration` query verbs; reads the `workflow.smart_zone_tokens` budget and `.planning/estimation-calibration.json`, both degrading to defaults rather than failing planning (#2630) |
 | `observability/event.cjs` | DispatchEvent shape factory for every Hub dispatch — traceId/parentTraceId/command/result/timestamp record consumed by DispatchLogger (#177, ADR-0174 P1.3/P1.4) |
-| `fallow-runner.cjs` | Fallow audit adapter for `/gsd-code-review`: binary resolution (`PATH` then `node_modules/.bin`), actionable missing-binary errors, and structural findings normalization |
+| `fallow-runner.cjs` | Fallow audit adapter for `/gsd-code-review`: binary resolution (`node_modules/.bin` then `PATH`), actionable missing-binary errors, and structural findings normalization |
 | `federated-config.cjs` | Defensive merge of capability-declared config slices into the loadConfig return value — ADR-857 phase 3b; exports `mergeFederatedConfig({ configSchema, isCentralKey, userConfig })` → `{ values, validKeys, warnings }`; live for migrated Capability keys that are atomically removed from the central config schema |
 | `frontmatter.cjs` | YAML frontmatter CRUD operations |
 | `gap-checker.cjs` | Post-planning gap analysis (#2493): unified REQUIREMENTS.md + CONTEXT.md decisions vs PLAN.md coverage report (`gsd-tools gap-analysis`) |