fix(async): close #813 audit gaps (#815/#817/#818) + confirm #816/#819/#820/#821

[zackees]

Summary

Final follow-up to #813's whole-app tokio runtime migration. PR #823 already landed the bulk of the conversion; this PR closes the last three gap-laden audit sub-issues and confirms the four that verified clean.

Verification + fixes by sub-issue

Verified clean (no work needed, closing on this PR):

• audit: sync code that could be async in fbuild-core + foundational crates (sub-issue of #813) #816 fbuild-core — subprocess.rs CRITICAL paths all async with _blocking shim escape hatch; response_file.rs async. Secondary findings (config loaders, library-select cache, install_status callback) are extension items beyond the core async migration — deferred.
• audit: sync code that could be async in fbuild-deploy (sub-issue of #813) #819 fbuild-deploy — Deployer::deploy + post_deploy_recovery async via #[async_trait]; all 4 per-platform deployers async; esptool/avrdude/picotool/pyocd all route through subprocess::run_command(...).await.
• audit: sync code that could be async in fbuild-build (sub-issue of #813) #820 fbuild-build — all 14 per-platform orchestrators async; compile_source async with direct .await on ZccacheService::compile; parallel.rs uses JoinSet/Semaphore; compile_many::run_stage2 async; compile_blocking deprecated.
• audit: sync code that could be async in fbuild-packages (sub-issue of #813) #821 fbuild-packages — shared OnceLock<reqwest::Client> HTTP layer; all Package::ensure_installed async via #[async_trait] across 31 impls; library_manager uses JoinSet; no reqwest::blocking::* references remain.

Gaps fixed in this PR:

• audit: sync code that could be async in fbuild-daemon (sub-issue of #813) #815 fbuild-daemon — daemon emulator handlers had 8+ sync std::fs::* calls running on the axum worker; export_artifacts_bundle (with sync I/O) was called outside spawn_blocking from build + deploy handlers. Fixed: converted 4 emulator handler files to tokio::fs::* (one helper flipped sync → async); wrapped 3 export_artifacts_bundle call sites in spawn_blocking. The broker IPC backend (broker/backend.rs) keeps its std::thread::spawn model intentionally — it's a sync IPC protocol, not on the tokio runtime by design.

• audit: sync code that could be async in fbuild-cli + fbuild-python (sub-issue of #813) #817 fbuild-python — 7 sync helpers still had ~30-line reqwest::blocking::Client blocks duplicated alongside Async* counterparts. Folded each into 1-2 line rt.block_on(async_version(...)) wrappers; extracted 4 shared async helpers; deleted unused intermediate sync wrappers. Zero reqwest::blocking::* references remain in fbuild-python/src/.

• audit: cross-cutting async/sync architecture (sub-issue of #813) #818 cross-cutting — last structural piece was BuildLog.sender: Option<std::sync::mpsc::Sender<String>> requiring a spawn_blocking bridge in the daemon's build handler. Flipped to tokio::sync::mpsc::UnboundedSender<String> and deleted the 10-line bridge task; the WebSocket forwarder now awaits log_rx.recv() directly.

Test plan

• `soldr cargo check --workspace --all-targets` — clean
• `soldr cargo clippy --workspace --all-targets -- -D warnings` — clean
• `soldr cargo test --workspace --no-fail-fast` — exit 0

Closes

• Closes audit: sync code that could be async in fbuild-daemon (sub-issue of #813) #815
• Closes audit: sync code that could be async in fbuild-core + foundational crates (sub-issue of #813) #816
• Closes audit: sync code that could be async in fbuild-cli + fbuild-python (sub-issue of #813) #817
• Closes audit: cross-cutting async/sync architecture (sub-issue of #813) #818
• Closes audit: sync code that could be async in fbuild-deploy (sub-issue of #813) #819
• Closes audit: sync code that could be async in fbuild-build (sub-issue of #813) #820
• Closes audit: sync code that could be async in fbuild-packages (sub-issue of #813) #821
• Closes audit: go fully async — whole-app tokio runtime sharing for tokio-console (meta) #813 (meta — all sub-issues addressed)

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📥 Commits

Reviewing files that changed from the base of the PR and between e1b4486 and 8ce4548.

📒 Files selected for processing (13)

• crates/fbuild-build/src/build_output.rs
• crates/fbuild-build/src/lib.rs
• crates/fbuild-core/src/build_log.rs
• crates/fbuild-daemon/src/handlers/emulator/avr8js_deploy.rs
• crates/fbuild-daemon/src/handlers/emulator/avr8js_npm.rs
• crates/fbuild-daemon/src/handlers/emulator/avr8js_web.rs
• crates/fbuild-daemon/src/handlers/emulator/qemu_deploy.rs
• crates/fbuild-daemon/src/handlers/operations/build.rs
• crates/fbuild-daemon/src/handlers/operations/deploy.rs
• crates/fbuild-python/src/async_serial_monitor.rs
• crates/fbuild-python/src/daemon.rs
• crates/fbuild-python/src/outcome.rs
• crates/fbuild-python/src/serial_monitor.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/async-followups-813

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