feat(arguments): bounded POSIX argument decoding

[zackees]

Implements #202. Adds optional command-arguments with facade-owned parse_posix, explicit semantic errors, 1 MiB input and 16384-word output limits, NUL rejection, and no shell execution or expansion. The exact existing shell-words 1.1.1 backend stays private and absent from default features. Generic quoting/escaping/Unicode/error/boundary tests live upstream.

Validation: missing-API RED then focused 3 tests GREEN; full command-arguments suite, strict all-target feature Clippy, default dependency isolation RED/GREEN and fmt passed. Windows x64 MSVC and macOS ARM64 cross-checks passed. Single-reviewer clud-review clean. App adoption is local and its real WASM build remains outstanding due intermittent Soldr transport failures; no end-to-end or release claim.

Stacked on #201 only to keep the FastLED integration checkout complete; the argument capability itself is independent of webviews. Must land upstream before the final published migration release.

Coordinated with FastLED/cli#242.

[zackees]

App adoption is now pushed in fastled-wasm commit 794a278 (PR242). Real Blink compilation passed with a fresh direct-cflags cache proving the parser was exercised. Full app Rust/Python checks and Clippy passed. Compiler validation exposed and fixed an app-specific staging-config bug (fastled-wasm#246). Real native viewer rendering remains independently unproven/failing under fastled-wasm#247; no Safari or release claim.

[zackees]

Closing as superseded: #220 replayed this stacked FastLED foundation onto current main after the #216 merge diverged, and it has landed.

Verified against origin/main (e0666e0): every file this PR's own commits add exists on main, and every distinctive added source/test line is present on main (one JSON doc-comment line was reworded). Merging this branch now would only reintroduce pre-refactor code that main has since replaced.

Reopen if something here is still missing from main.