feature: ls walk glob filesystem builtins

[danieljohnmorris]

Summary

Three new Result-typed builtins for directory enumeration, closing the categorical gap personas have hit repeatedly: rd exists for single files but there was no way to list a directory or recursively find files matching a pattern (a basic facility every shell and scripting language ships).

• ls dir > R (L t) t - non-recursive listing, filenames only, sorted lexicographically. Empty dir returns [], missing dir / permission denied surface as Err.
• walk dir > R (L t) t - depth-first recursive walk, paths relative to dir with forward slashes (deterministic across OSes). Symlinks not followed.
• glob dir pat > R (L t) t - shell-style filter. */?/[abc] within a segment, ** recursive across segments. No matches returns [], not Err.

All three are tree-bridge eligible so VM and Cranelift inherit them through the same path rd/rdb/sleep already use - no new opcodes, no Cranelift helpers, no AOT touch.

Why

Originating: filesystem-walk persona reports in ilo_assessment_feedback.md. Token-cost framing: walk dir is 8 chars vs the alternative of asking the user to pre-compute the file list and pass it as an arg, or hand-rolling a recursive list-pass with rd on each file. Plus the matcher in glob collapses the most common shape (find . -name '*.ext') to a single call.

Repro before / after

Before (no way to list a directory at all):

```
$ ilo 'main>n;len ls "."'
ILO-T004: undefined variable 'ls'
```

After:

```
$ ilo 'main>R (L t) t;ls "examples"'
[02-with-dependencies.ilo, at-float-index.ilo, ...]

$ ilo 'main>R (L t) t;glob "examples" "**/*.ilo"'
[02-with-dependencies.ilo, ..., window-listview-perf.ilo]
```

What's in the diff

Four commits, one logical change per commit:

1. free the ls binding name ahead of fs builtins - rename four pre-existing ls= bindings (two test fixtures, one example, plus the bindings inside the builtin-binding-name-rename.ilo example which itself uses ls as a local) to non-builtin names. Needed before ls lands as a reserved short name.
2. add ls / walk / glob filesystem builtins - Builtin enum entries + from_name/name/ALL/round-trip-tests in src/builtins.rs, signatures in src/verify.rs, tree-walker impl + helper walk_collect + helper glob_match in src/interpreter/mod.rs, tree-bridge eligibility in src/vm/mod.rs.
3. test ls / walk / glob across every engine - 11 cross-engine tests in tests/regression_fs_builtins.rs using a per-test tempdir fixture, plus examples/fs-builtins.ilo for the engine harness (uses the repo's own examples/ dir as a stable fixture).
4. sync docs for ls / walk / glob - SPEC.md builtins table + reserved-names list updated, ai.txt and skills/ilo/SKILL.md regenerate from SPEC via build.rs.

Test plan

• cargo test --release --features cranelift - whole suite green
• cargo fmt --check clean
• cargo clippy --release --features cranelift -- -D warnings clean
• tests/regression_fs_builtins.rs - 11 tests across --run-tree, --run-vm, --jit
• tests/examples_engines.rs - new examples/fs-builtins.ilo runs through every engine
• tests/regression_reserved_names_doc.rs - SPEC reserved-names list matches Builtin::ALL

Follow-ups

• Site doc (docs/builtins/data-io.md in the ilo-lang/site repo) edit prepared at /Users/dan/code/ilo-lang/site-docs-refresh/src/content/docs/docs/builtins/data-io.md but couldn't push - the site main checkout is not present on this machine; needs landing in a separate PR over there.

[codecov]

Codecov Report

❌ Patch coverage is 78.78788% with 42 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/interpreter/mod.rs	77.04%	42 Missing ⚠️

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