feat(runtime,fs): emit FsChanged for in-shell VFS writes - #1

Summary

FsChanged(path) was emitted only by the explicit WriteFile protocol command. In-shell writes — echo > f, touch, mkdir, cp, rm, tee, redirections — ran through the executor and emitted just [Exit], even though the file was really written (read-back confirms). The protocol doc promised notification "when scripts touch the FS", but the standalone/Pyodide paths didn't deliver it.

This adds a single chokepoint at the VFS layer:

• wasmsh-fs: new FsChangeLog — an Rc<RefCell<…>>-backed, insertion-ordered, de-duplicating set of mutated paths, exposed via a defaulted Vfs::change_log() accessor. MemoryFs and EmscriptenFs record the path on every mutating op (write-intent open, write_file, open_write_sink, create_dir, remove_file, remove_dir). The OpfsFs stub inherits the None default.
• wasmsh-runtime: drain_fs_change_events emits one FsChanged(path) per distinct path at each event-batch boundary (poll_active_run Done + Yield, finish_idle_signal_exit), placed after I/O/diagnostics and before Exit. Internal scratch paths (/tmp/_wasmsh_*, /tmp/_proc_subst_*) are suppressed. handle_write_file_command is unified onto the same drain path.
• Docs: docs/reference/protocol.md updated to match; design recorded in ADR-0032 (status: Proposed).

Semantics: per-path, de-duplicated per run, first-touch order. Scope note: files written by the in-process Python interpreter go through Emscripten libc directly (not the shell VFS), so they are not reported via FsChanged.

This is purely additive to the event stream — hosts already handling FsChanged from WriteFile need no changes.

Test plan

Per ADR-0020 (E2E-first); FsChanged isn't expressible in the TOML differential oracle (real bash emits no such event).

• cargo test -p wasmsh-fs — change-log record/dedup/order/read-only unit tests
• cargo test -p wasmsh-runtime — protocol tests: redirect, mkdir/touch/cp/rm per-path, dedup, read-only emits nothing, heredoc scratch suppressed
• cargo fmt --all, cargo clippy (incl. --features emscripten), cargo check --workspace --all-targets
• TOML suite (wasmsh-testkit suite_runner) green — no regression
• just test-e2e-standalone (added cases in e2e/standalone/tests/file-ops.spec.ts) — requires wasm-pack build
• just test-e2e-pyodide-node (added case in e2e/pyodide-node/tests/protocol-parity.test.mjs) — requires emcc/Pyodide build

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