fix(realfs): avoid blocking async runtimes

[chaliy]

What changed

RealFs async backend operations now use Tokio filesystem APIs throughout, preventing host filesystem work from blocking current-thread runtimes. Async construction is available through RealFs::open; the synchronous RealFs::new migration shim is deprecated.

Async JavaScript mount paths use the nonblocking constructor. Existing synchronous Rust builder, Python, and JavaScript factory surfaces remain operational during migration.

Why

Synchronous path resolution and metadata work inside async RealFs operations could stall or deadlock a single-worker Tokio runtime. The deterministic regression combines captured custom-builtin pipeline output with a FIFO-backed RealFs path.

Before / After

Before: the current-thread regression timed out after 500 ms because synchronous FIFO opening blocked the only runtime worker; the same test passed on a multi-thread runtime and masked the defect.

After: both current-thread and multi-thread pipeline tests complete in about 60 ms, while containment, symlink, read-only, and read-write semantics remain covered.

Risk

• Medium
• RealFs path-resolution timing and the recommended Rust constructor change. The deprecated constructor preserves source compatibility; focused security and cross-language tests cover existing behavior.

Validation

• just pre-pr
• RealFs unit tests: 39 passed
• RealFs integration/runtime tests: 38 passed
• Node tests: 520 passed; TypeScript build/typecheck passed
• Python ABI3 wheel built; Python 3.9, 3.12, and 3.14 sync/async RealFs smoke tests passed
• WASM build/tests and parallel-execution benchmark passed during investigation

Checklist

• Tests added or updated
• Backward compatibility considered

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	4e16a71	Commit Preview URL Branch Preview URL	Jul 25 2026, 11:18 PM