feat(js): expose real filesystem mounts with per-mount readOnly support (#1077)

## Summary

- Add `MountConfig` napi object and `mounts` option to `BashOptions` for
configuring real filesystem mounts at construction time (host path,
optional VFS path, optional readOnly flag defaulting to true)
- Add `mountReal(hostPath, vfsPath, readOnly?)` and `unmount(vfsPath)`
runtime methods to both `Bash` and `BashTool` classes
- Mount configs stored in `SharedState` and re-applied on `reset()` for
consistency
- Enable `realfs` feature in bashkit-js Cargo.toml dependency
- Fix pre-existing broken code: add missing `shared_state_from_opts`
function and fix stale `build_bash` reference in `BashTool::reset`
- Add 3 integration tests for runtime mount/unmount (readonly,
readwrite, unmount verification)

Closes #1066

## Test plan

- [x] `cargo check -p bashkit-js` compiles cleanly
- [x] `cargo fmt` passes
- [x] 3 new `runtime_mount_*` / `runtime_unmount` tests pass in
`realfs_tests.rs`
- [ ] CI green

Author: chaliy

crates/bashkit-js/Cargo.toml

@@ -14,7 +14,7 @@ repository.workspace = true
 crate-type = ["cdylib"]
 
 [dependencies]
-bashkit = { path = "../bashkit", features = ["scripted_tool", "python"] }
+bashkit = { path = "../bashkit", features = ["scripted_tool", "python", "realfs"] }
 napi = { workspace = true }
 napi-derive = { workspace = true }
 serde = { workspace = true }