fix(test-emu): fail-fast and corruption-aware avr8js npm cache

[zackees]

Closes #86.

fbuild test-emu --emulator avr8js failed with Node ERR_MODULE_NOT_FOUND when ~/.fbuild/cache/avr8js-node/node_modules/avr8js/ was missing or corrupt. Root cause: ensure_avr8js_npm's exists() check accepted a directory that lacked package.json.

Changes

• Integrity marker: require node_modules/avr8js/package.json as the proof of a non-corrupt install. If the dir exists but the marker doesn't, wipe and reinstall.
• Error messages: when npm is not on PATH or install exits non-zero, capture both stdout and stderr and name the cache dir.
• New env var FBUILD_REFRESH_EMU_CACHE=1 forces reinstall regardless of marker state (accepts 1|true|TRUE|yes|YES).
• Fail-fast logging before spawning node when the cache state is still bad.

Node-spawn audit

Two spawn sites: find_node() (probes node --version, no avr8js needed) and run_avr8js_headless(). Both callers (deploy_avr8js, Avr8jsRunner::run) already called ensure_avr8js_npm() first — no bypass. The real gap was the integrity check.

Test plan

• uv run cargo test -p fbuild-daemon --lib — 107 passing
• uv run cargo clippy --workspace --all-targets -- -D warnings
• uv run cargo fmt --all

Deferred

• CLI flag --refresh-emu-cache plumbed through TestEmuRequest (env var works but needs daemon restart)
• find_node() still returns a bare node/node.exe PathBuf rather than an absolute resolved path — minor race window between probe and spawn if PATH changes

🤖 Generated with Claude Code

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📒 Files selected for processing (1)

• crates/fbuild-daemon/src/handlers/emulator.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/avr8js-cache-hardening-86

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