✨ feat: add a deterministic contextual <Glob> component

[taras]

Why

A document decides what to work on by looking at what is there — repository
instructions, test fixtures, source files, workflow inputs. Until now it had to
shell out to find or ls and parse the result, which is neither deterministic
nor something an author can assert against.

Closes #191. Follows #176 (schema-validated returns), #189 (<TempDir>), and
#190 (<File>).

What changes

<Glob> is a new built-in value component.

Before:

```sh exec
find . -name 'AGENTS.md'
```

— output that varies by host, by locale, and by directory order, with no way to
branch on it.

After:

<Glob include={["**/AGENTS.md"]} as="instructionPaths" />

— a string[] of relative POSIX paths, deduplicated and sorted by code point.

Because patterns are relative to the contextual Env.cwd, the three filesystem
components become a pipeline without any of them knowing about the others — a
path <Glob> returns is a path <File> can read:

<TempDir>
<File path="docs/guide.md">Guide</File>
<Glob include={["docs/**/*.md"]} as="docs" />
<Each in={docs} let="path">
<File path={path} />
</Each>
</TempDir>

How it works

<Glob include exclude as> → check patterns → stat Env.cwd → API.Fs.glob → files only → dedupe → sort

Patterns are checked before the filesystem is touched, Env.cwd is checked
before traversal, and matching itself belongs to the Fs Api — the component adds
no syntax of its own.

Review guide

Start with: packages/core/src/components/Glob.test.md — the public matching
contract, entirely in Markdown.

Then review:

1. specs/executable-mdx-spec.md §6.14 — props, dialect, ordering,
   normalization, symlinks, failures
2. packages/core/src/components/Glob.ts — the component
3. packages/runtime/apis.ts — the two API.Fs.glob fixes below
4. packages/core/tests/glob-component.test.ts — Tier GB
5. packages/core/src/components/fs-diagnostics.ts + File.ts — the extracted
   errno allowlist
6. site/routes/docs/components.tsx — website section

Look carefully at: the traversal in descend(). It is where containment,
cancellability, and exclusion pruning all live.

Two defects in API.Fs.glob

Neither of the issue's acceptance criteria for exclude or traversal failure
could hold without fixing these, so they are in scope.

exclude never matched anything. It was passed to @effectionx/fs's
walk({ skip }), which tests patterns against the absolute entry path — so
globToRegExp(".git/**"), anchored ^\.git\/…$, could never match
/tmp/x/.git/config. Matching now happens on the relative POSIX path.

A traversal failure was an uncaught rejection. walk produces entries from
a spawned task, so a readdir that threw tore down the surrounding scope
instead of throwing at the call site — no caller could catch or report it.
Verified before the change: a chmod 000 subdirectory crashed the process with
Uncaught (in promise) Error: EACCES past a try/catch around the call.
Traversal is now a plain recursive generator, which also makes every directory
read a cancellation point.

What must stay true

• Traversal never leaves Env.cwd — enforced by never following a symlink
  (a link is not a directory to descend, and not a file to return), checked by
  GB15/GB15b/GB16/GB16b.
• Traversal cannot cycle — same mechanism, checked by GB15c, which fails by
  timing out rather than hanging.
• A listing is host-independent — dedupe plus code-point sort, checked by
  GB19 and by the ordering/dedup tests in Glob.test.md.
• No absolute path reaches a diagnostic (§1.2) — every Api call is wrapped
  and the errno selects an allowlisted phrase; checked by GB12, GB12b, GB13,
  and four adversarial GB14 cases including an externally thrown GlobError.
• An empty result means "no such files" — so a pattern that cannot match
  anything fails instead, checked by GB6/GB7/GB8.
• Pruning never changes the answer — a subtree is skipped only when a
  trailing /** provably covers every descendant; every other exclusion walks
  and filters per file. Checked by Tier GA, which counts directory reads because
  a result set cannot tell a correct filter from an over-eager prune.

How to verify it

• Glob.test.md (27 assertions) proves the matching contract with no
  JavaScript, and fails if include/exclude precedence, ordering, dedup,
  normalization, hidden-file semantics, or files-only filtering regress.
• GB9 proves an uncompilable pattern is reported as an authoring error and
  fails if a raw RegExp message ("Unterminated character class") leaks.
• GB7 proves refusing only a leading .. is sufficient: a .. further
  along (docs/../../../etc/passwd) matches nothing because traversal never
  produces such a path.
• GB17 proves traversal is cancellable mid-walk — the second directory read
  suspends, the third is never read, and the document gets nothing.
• GB18/GB18b prove replay: a completed root restores the array without
  searching; a partial journal searches again and sees newly added files.
• GA2/GA2b/GA3 prove exclusions are read as written: foo/* keeps
  foo/deep/keep.md and still walks foo, and vendor alone removes nothing.
  They fail if a directory match is ever taken as proof about its descendants.
• GA4/GA4b/GA5 prove pruning is sound and still happens: .git/** and
  **/node_modules/** read nothing below them, while the equally-total **/*
  walks and filters — same answer, different work.

Verified on Deno 2.9.1 (CI-pinned): lint, check, test (215 passed /
1723 steps), check:jsr all clean. Also test:node (1698/0), test:bun
(1698/0), site check, and ./dist/xmd test packages/core/src from the
compiled binary.

Scope

Included

• <Glob>, registered as a built-in
• The three API.Fs.glob fixes: relative-path exclusion matching, catchable
  traversal failures, and sound subtree pruning
• Tier GA, which tests glob() directly rather than through any caller
• The shared errno allowlist, extracted from File.ts
• Spec §6.14 and website documentation

Intentionally unchanged

• resolveTestTarget is the only other glob caller. It passes no exclude
  and filters on isFile, so its behavior is identical — confirmed by the
  test-target suite and by the compiled binary still discovering every
  document under packages/core/src.
• API.Fs.glob keeps returning { path, isFile } and keeps reporting symlinks
  with isFile: false. Baking "files only" into the Api would be the
  component's opinion, not the Api's.
• Following symlinks is still not offered. @effectionx/fs exposes
  followSymlinks but guarantees neither root containment nor cycle detection.

New abstractions

• components/fs-diagnostics.ts exists because the errno-to-phrase allowlist is
  now needed by two components with the same constraint; duplicating fourteen
  entries would let them drift.
• GlobError exists to mark the component's own failures, on the same terms as
  FileAccessError.
• Each new abstraction has multiple concrete uses or a clear justification.
• No speculative functionality is included.

Risks and limitations

• Not a sandbox. <Glob> never follows a symlink, so nothing it reads is chosen
  by one — but a directory that is real when read could be replaced afterwards.
  Containment independent of observed filesystem state is Contain filesystem access at the provider boundary #227.
• Subtree pruning is a syntactic decision: only a trailing /** earns it. A
  pattern that happens to cover a whole subtree by other means — a character
  class enumerating every child, say — is walked and filtered instead. That
  costs reads and never costs correctness, which is the direction to be wrong
  in.

Scope confirmation

• Every changed file supports the purpose described above.
• Unrelated cleanup and formatting changes are excluded.
• Generated or mechanical changes are clearly identified.
• The description matches the final diff and test results.

[github-actions]

PR #229: ✨ feat: add a deterministic contextual component

10 files, +1899 / -93

Scope

🔴 PR has 1992 lines changed. Split into focused PRs.

🟡 1992 lines changed. PRs under 400 receive more thorough review.

Structural

✅ No structural bloat detected.

Slop

✅ Slop indicators look low.

Static Analysis

✅ Oxlint found no issues.

Correctness

No extraneous code patterns detected.

[github-actions]

Found 2 redundant comments. Inline suggestions to remove them below.

[github-actions]

Redundant comment — restates what the code does.

Suggested change

// depend on the locale the host happens to be configured with.

[github-actions]

Redundant comment — restates what the code does.

Suggested change

// stays under the root and cannot cycle.