Module cache 

Lazy generation cache for .wasm modules.

Two kinds of artefacts land here:

1. Per-source compilations — e.g. a Rust snippet → WASM goes into runtimes/rust/<source-hash>.wasm. Idempotent: same source text never recompiles.
2. Drop-in interpreters — once Steel-WASM / QuickJS-wasi ship, a user can place runtimes/<lang>.wasm here and the registry discovers it (M2 follow-up).

Path resolution:

• Linux/BSD → $XDG_CACHE_HOME/outl/runtimes/ or ~/.cache/outl/runtimes/
• macOS → ~/Library/Caches/outl/runtimes/
• Windows → %LOCALAPPDATA%\outl\runtimes\

Delegated to the dirs crate so we follow OS conventions instead of hard-coding paths.

Functions

cache_dir

Root cache directory: <os cache>/outl/runtimes/.

cache_path_for_source

Path where a compiled artefact for (language, source) should land. The filename is <sha256-of-source>.wasm; collisions are vanishingly unlikely and the hash also doubles as a cache key for “is this source the same as last time?”.

is_fresh

Does the file at path exist and is non-empty? Used by lazy-gen runtimes to decide “skip the compile, just read the bytes”.