Adds five new dylints, completing the #826 audit:

- ban_process_exit_outside_main — `std::process::exit` only in
  `crates/*/src/main.rs` and `crates/*/src/bin/*.rs`. Skipping
  destructors leaks temp files, kills containment guards, and
  truncates in-flight HTTP/WS responses. Legacy CLI subcommand
  dispatchers (build/deploy/dispatch/lnk/pio/purge/reset/serial_probe/
  show) exempted via src/allowlist.txt.
- ban_unwrap_in_daemon_handlers — `.unwrap()` banned in
  `crates/fbuild-daemon/src/handlers/**`. Test files (*tests*.rs) and
  `#[cfg(test)]` modules are exempt; lint walks owner chain to detect
  the cfg(test) ancestor. Three legacy production sites
  (websockets.rs, operations/build.rs, operations/deploy.rs) are
  file-level allowlisted with follow-up notes.
- cli_no_build_deploy_direct_use — `fbuild-cli` is a thin HTTP client
  to the daemon; importing `fbuild_build::*` / `fbuild_deploy::*` is
  reserved for diagnostic subcommands (args/bloat_lookup/graph_cmd/
  symbols_cmd/compile_many/reset).
- require_multi_thread_flavor_when_spawning — `#[tokio::test]` on an
  async fn that calls `tokio::spawn` in its body must specify
  `flavor = "multi_thread"`, otherwise spawned tasks deadlock under
  the default `current_thread` flavor. Two legacy files
  (handlers/websockets_tests.rs, packages/install_lock.rs)
  allowlisted for follow-up migration.
- ban_std_sync_mutex_in_async — `std::sync::Mutex` /
  `std::sync::RwLock` banned in `crates/fbuild-daemon/src/**` and
  `crates/fbuild-serial/src/**`. Holding the guard across `.await`
  starves the Tokio worker; a panic poisons the lock for every
  subsequent caller. Test files and `#[cfg(test)]` modules exempt;
  existing synchronous-discipline uses (context.rs, device_manager.rs,
  status_manager.rs, handlers/operations/common.rs, manager.rs)
  allowlisted with inline justification.

All five lints follow the existing pattern from PR #833:

- Own crate under dylints/, own rust-toolchain.toml pin
  (nightly-2026-03-26), trailofbits/dylint at git rev 4bd91ce…
- Workspace excludes the crate so the stable workspace build is
  unaffected
- File-path-scoped detection via cx.sess().source_map() with
  slash-normalized matching
- File-level allowlist.txt with inline justifications
- Tests covering scope detection, allowlist suffix matching, and
  entry-point exemptions

Three additional items from #826's Tier 4 are not lints — they're
audit / process work tracked as separate follow-up issues:

- Mocks-as-primary-test-surface audit (recommend CodeRabbit rules,
  not dylint)
- Ignored-test bitrot policy (recommend weekly grep-count to
  tracking issue)
- Initializer-order lints (ban_env_var_set_after_import,
  require_oncelock_install_before_use) — need global flow analysis
  out of scope for this PR

Test plan:
- soldr cargo check --workspace --all-targets ✓
- soldr cargo clippy --workspace --all-targets -- -D warnings ✓
- soldr cargo test --workspace --no-fail-fast ✓ (all pass)
- `cargo dylint --all` validation deferred to CI on Ubuntu — the
  Windows host's Chocolatey rust shadow blocks local invocation
  (filed as zackees/soldr#1059)

See #826 for the original gotcha-sweep tracking issue
and PR #833 for the first five lints this builds on.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>