fix(fs): validate mount paths with POSIX semantics on Windows

[chaliy]

Summary

MountableFs::mount validated paths with Path::is_absolute, which on Windows requires a drive prefix (C:\…) and rejects POSIX-style absolute paths like /workspace. Bashkit's VFS is always POSIX-style on every host, so this broke bash.mount("/workspace", fs) on Windows.

This surfaced after the v0.2.0 release as the publish-only test failure:

vfs › filesystem external roundtrip mounts into bash
Error: io error: mount path must be absolute
  at Bash.mount (...wrapper.ts:855:19)
  at vfs.spec.ts:159

The interop FS feature in #1353 exercised mount() from JS for the first time, and the JS publish workflow runs the full Windows × Node 20/22/24 matrix that PR CI doesn't.

Why this regressed silently

PR CI runs Rust tests on ubuntu-latest only. Windows coverage exists exclusively in publish-js.yml, which only runs on release: published. So the bug rode all the way to the v0.2.0 GitHub Release before it was observed.

Fix

• Use Path::has_root instead of Path::is_absolute. has_root returns true for any leading / on both Unix and Windows.
• Move the validation ahead of normalization so the caller's intent (absolute vs. relative) is checked against the original input — normalize_path always returns a rooted path, which would have masked the check entirely had we left it after.
• Extract the check into a small is_posix_absolute helper so the platform-portability invariant is unit-testable in isolation.

Test coverage

Added in crates/bashkit/src/fs/mountable.rs:

• test_is_posix_absolute_accepts_root_paths — /, /workspace, /data/sub
• test_is_posix_absolute_rejects_relative_paths — relative, relative/path, ./foo, ""
• test_mount_accepts_posix_absolute_path_on_any_host — regression test asserting mount("/workspace", …) and mount("/data/sub", …) succeed (post-fix this passes on Windows; pre-fix it would fail there)
• test_mount_rejects_relative_path — error path still rejects relative inputs with an absolute message

PR CI verifies the Linux behavior is unchanged. Windows behavior will be verified by the JS publish workflow's vfs.spec.ts once v0.2.1 is cut (or by re-dispatching publish-js.yml against v0.2.0 after this lands on main).

Test plan

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace (84 suites green)
• CI green on PR
• After merge: re-dispatch publish-js.yml for v0.2.0 and confirm Windows tests pass + npm publishes

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
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