Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@crates/fbuild-build/src/build_fingerprint/fast_path.rs`:
- Around line 136-149: add_default_watches currently skips adding the "dep_libs"
root when fast_path_watch returns None, causing missing-root state to be lost;
update add_watch_root (or fast_path_watch) so it always records a watch entry
even when the directory is absent: call fast_path_watch(cache_name,
&self.build_dir, root) and if it returns None, push a placeholder watch that
encodes "missing root" (e.g. Watch::missing_root or similar) into self.watches
instead of skipping; this preserves the root in add_default_watches for
"project" and "dep_libs" and lets the hash/check layer represent the absent-root
state.

In `@crates/fbuild-build/src/esp32/orchestrator.rs`:
- Around line 1256-1264: The call to
crate::build_fingerprint::persist_fast_path_success with FastPathPersistInputs
(using metadata_hash, link_result.size_info, params.watch_set_cache,
compiler_cache) must only run after the entire build (link/convert and any final
artifact writes) has completed successfully; move or gate this call so it
executes only on the successful build path (i.e., after the final link/convert
result is confirmed OK and no errors were returned), rather than unconditionally
where it currently runs.
- Around line 262-275: The fast-path contract currently built by
FastPathContract::for_project_outputs is missing the ESP32 side-output
boot_app0.bin, so add the corresponding fast_app0 (or similarly named artifact
token you already use elsewhere) into the list passed to
FastPathContract::for_project_outputs alongside fast_elf, fast_bin, fast_boot,
fast_parts, and fast_compile_db; ensure the symbol you add matches the variable
used when the cold path copies boot_app0.bin (so the contract and the copy refer
to the same artifact name) so warm fast-path hits will require/own boot_app0.bin
as well.

---

Nitpick comments:
In `@crates/fbuild-build/src/renesas/orchestrator.rs`:
- Around line 62-71: The helper expected_fast_path_artifacts and its duplicate
test should be centralized: move the logic into build_fingerprint::fast_path as
a function that accepts the firmware filename(s) (e.g.,
"firmware.bin"/"firmware.hex") plus build_dir and project_dir, and return the
same tuple (elf, firmware, compile_db) using
CompileDatabase::expected_output_path; then update the orchestrator-specific
expected_fast_path_artifacts references to call the new
build_fingerprint::fast_path with the appropriate filename(s) and remove the
duplicated unit test
test_expected_fast_path_artifacts_follow_compile_db_location from
per-orchestrator modules, keeping a single shared test alongside the new helper.