enhance(#3619): ratchet the platform seam with local/no-private-binary-resolution (epic #3411 Phase 3) (#3636)

* chore(#3619): ratchet the platform seam with local/no-private-binary-resolution

Epic #3411 Phase 3, the ratchet. Scope revised with maintainer approval and
recorded on the issue: the epic's literal ask was a rule rejecting a bare-name
spawn outside the seam. Surveyed at ac1b6d679, ~30 such sites exist and none is
a defect — git, gh and npm ship native .exe that CreateProcess resolves unaided,
and the rest are POSIX-only tools. ADR-1703 rules 2 and 3 forbid grandfathering
and escape hatches, so a literal rule would be unsuppressable and would force
rewriting 30 correct calls.

The epic's actual thesis was four private RESOLVERS, not four bare spawns. So
the rule flags re-implementing resolution: reading PATHEXT in any casing from
any object, and a hardcoded list carrying two or more of .exe/.cmd/.bat/.com —
precisely the shapes fallow-runner's candidateNames and gsd-tools' PATHEXT
string had before Phases 1 and 2 deleted them.

Three boundaries were arrived at rather than assumed:

  two-or-more   a single .endsWith('.cmd') is a classification, not a candidate
                set; runtime-hooks-surface derives .cmd shim paths that way
  boundary-aware  a naive substring test flags .execute and .compacting, caught
                on src/host-integration.cts before it could become a false
                positive nobody could suppress
  suffix-anchored  the seam exemption matches src/shell-command-projection.cts
                exactly; a substring match would also exempt the dispatch test
                file. Case I9 pins it.

PATH scans are deliberately NOT flagged — membership checks (bin/install.js)
are indistinguishable from resolution scans, and an unsound rule in a
zero-escape-hatch architecture is worse than no rule.

To make the ratchet strict with no carve-out, resolveExecutableBinary gained
pathOverride: search THIS PATH, read everything else including PATHEXT from the
ambient environment. resolveFallowBinary now supplies its own search path
without hand-threading PATHEXT, which would itself have been a private read.
The three alternatives were all worse: exempting the file is grandfathering,
exempting the AST shape is a carve-out every future caller must replicate, and
dropping the pass-through would silently ignore a user's real PATHEXT — buying
a lint rule with a correctness regression.

eslint-rules/** is outside the rule's globs rather than exempted, because
portability-vocab.cjs owns the extension set. scripts/**/*.cjs got its own block
so that exclusion does not leave a hole in the ratchet.

Started green with nothing suppressed. Proven able to fail: a fixture with both
signals reports two errors.

Refs #3411

* fix(#3619): close the PATHEXT destructuring evasion and correct two overclaims

Adversarial review found a trivial evasion of the rule's primary signal: the
visitor only handled MemberExpression, so

  const { PATHEXT } = process.env
  const { PATHEXT: exts } = process.env
  const { Pathext } = opts.env

were all unflagged. That is a common idiom, not an exotic bypass. An ObjectPattern
visitor now catches it in every form — renamed, any casing, any receiver, string
keys — while leaving a computed key alone, since it is not statically decidable.
I10-I13 pin the invalid forms and V9/V10 pin PATH and the computed key.

Two overclaims corrected, both mine:

Standards review proved the docs were factually wrong. Both the ADR amendment and
the CONTEXT.md entry asserted that tests/shell-command-projection-dispatch.test.cjs
is still linted by this rule. It is not — the rule's surface is src, gsd-core/bin,
scripts and hooks, and tests/** is deliberately outside it because test setup
legitimately assigns process.env.PATHEXT (fallow-runner's P3 does exactly that).
The suffix-vs-substring distinction is therefore proven by RuleTester case I9
feeding a synthetic filename, NOT by real coverage of that file. Both documents now
say so.

The rule's own docstring claimed the seam exemption matches the seam path
'exactly'. It is a suffix match, so a nested foo/src/shell-command-projection.cts
would also be exempt. Suffix matching is kept — it is how sibling rules resolve
paths and the nested case does not exist — but the docstring now states the
boundary rather than overstating the precision.

The evasion fix was verified by executing eslint against both destructuring forms
in scripts/, not by inspection. Probe: 31/31.

Refs #3411

* chore(#3619): backfill changeset pr number 3636

---------

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

src/shell-command-projection.cts

@@ -754,12 +754,26 @@ function _buildVerbatimCmdLine(target: string, args: string[]): string {
  * with plain `fs.writeFileSync` and never set an exec bit (the repo bans
  * `chmod` in tests), so every one of them would resolve to `null` on POSIX.
  *
+ * **`opts.pathOverride`** — "search THIS PATH, but read everything else —
+ * PATHEXT included — from the ambient environment." When set (`!== undefined`),
+ * the PATH search segments come from splitting `pathOverride` on `path.delimiter`
+ * instead of from `env.PATH`; `pathOverride: ''` means an explicit EMPTY search
+ * path (zero segments), never a fallback to `env.PATH` — use `!== undefined`,
+ * not truthiness, to tell "caller supplied a PATH string" apart from "caller
+ * supplied nothing". `opts.prependPaths` still comes first. PATHEXT resolution
+ * is UNAFFECTED by this option — it still reads from `env` (which defaults to
+ * `process.env`) exactly as it does when `pathOverride` is omitted. This exists
+ * so a caller that already has its own search path in hand (e.g.
+ * `resolveFallowBinary`'s `envPath`) does not have to hand-thread PATHEXT
+ * alongside it — a private PATHEXT read is the very shape
+ * `local/no-private-binary-resolution` forbids outside this seam.
+ *
  * @returns the resolved path, or `null` when nothing matched. Callers fall back to
  *   the declared name on `null` so a genuine ENOENT still surfaces (#3086).
  */
 export function resolveExecutableBinary(
   name: string | null | undefined,
-  opts: { platform?: string; env?: NodeJS.ProcessEnv; prependPaths?: string[]; requireExecutable?: boolean } = {},
+  opts: { platform?: string; env?: NodeJS.ProcessEnv; prependPaths?: string[]; requireExecutable?: boolean; pathOverride?: string } = {},
 ): string | null {
   if (!name) return null;
   const requireExecutable = opts.requireExecutable ?? false;
@@ -768,7 +782,8 @@ export function resolveExecutableBinary(
     return _isFile(name, requireExecutable, platform) ? name : null;
   }
   const env = opts.env ?? process.env;
-  const pathSegments = String(_envGet(env, 'PATH') || '').split(path.delimiter).filter(Boolean);
+  const rawPath = opts.pathOverride !== undefined ? opts.pathOverride : (_envGet(env, 'PATH') || '');
+  const pathSegments = String(rawPath).split(path.delimiter).filter(Boolean);
   const segments = [...(opts.prependPaths ?? []), ...pathSegments];
 
   if (platform !== 'win32') {