feat(vfs): lazy file content loading for InMemoryFs

[chaliy]

Problem

InMemoryFs stores all file content eagerly as Vec<u8> at mount time. When mounting large file sets (e.g. a repository with thousands of files), all content is loaded into memory upfront, even though scripts typically touch only a fraction of files.

Current implementation

crates/bashkit/src/fs/memory.rs:172-189 — FsEntry::File always stores content: Vec<u8>. The add_file() method at line 772 immediately calls content.to_vec().

JS bindings (crates/bashkit-js/src/lib.rs:151-164) accept files: Option<HashMap<String, String>> — string content only, no callbacks.

Python bindings similarly accept eager string content.

Proposed approach

Add a lazy variant to FsEntry:

enum FsEntry {
    File { content: Vec<u8>, metadata: Metadata },
    LazyFile { loader: Box<dyn FnOnce() -> Vec<u8> + Send + Sync>, metadata: Metadata },
    // ... existing variants
}

Key behaviors:

• Loaded on first read: When a lazy file is read, the loader is called and the entry is replaced with a regular File
• Skipped if written before read: If a script writes to a lazy file before reading it, the loader is never called
• Cached after first load: Once loaded, behaves identically to a regular file
• Stat without loading: stat() should return metadata without triggering the load

Binding support

• JS: Accept files values as either strings or callback functions
• Python: Accept callables alongside string content
• Rust: builder.mount_lazy(path, || read_file_content(path))

References

• Inspired by just-bash #115 which added functions as file content suppliers for InMemoryFs