[zackees]

Fixes #1753.

Root cause

The warm-build fast path hashes a watch set of project files and reuses the
previous firmware.bin when the hash is unchanged. Both walks
(hash_watch_set, hash_watch_set_stamps) used WalkDir::new(root) without
follow_links, and walkdir reports a symlink entry as neither a file nor a
directory. So for

<proj>/lib/FastLED -> /home/niteris/dev/fastled2

the entire library was invisible to the fingerprint. A git checkout that
rewrote src/platforms/esp/32/core/fastspi_esp32_idf.h left
build_fingerprint.json byte-identical, the fast path hit, and the deploy
shipped the previous build's artifacts — the two branches measured
byte-for-byte identical firmware until --clean forced a full build. That is
the exact symptom in #1753, including why a plain lib/ (non-symlinked)
library was unaffected: its headers were already in the watch set.

check_with_stamps is the production branch (cancel... inputs.compiler_cache
is None for every orchestrator), so this is the path that produced the stale
firmware.

Change

• One shared watched_files(watch) helper for both hashes, with
  .follow_links(true).
• Cycles are safe: walkdir yields an error for a loop and .flatten() drops
  it; covered by test_hash_watch_set_stamps_tolerates_a_symlink_loop.
• Excludes still prune by directory name, so a .git inside a link target
  stays out of the hash.
• Symlinked files (e.g. a symlinked TU) are now hashed too.

Tests (fail before the fix, pass after)

Added to crates/fbuild-build-engine/src/build_fingerprint/mod.rs:

• test_hash_watch_set_stamps_sees_edits_under_a_symlinked_library
• test_hash_watch_set_sees_edits_under_a_symlinked_library
• test_hash_watch_set_stamps_hashes_a_symlinked_source_file
• test_hash_watch_set_stamps_tolerates_a_symlink_loop

All three "sees edits" tests fail on origin/main with identical
before/after hashes and pass with the change.

Gates

• soldr cargo test -p fbuild-build-engine --lib → 504 passed, 0 failed, 4 ignored
• soldr cargo test -p fbuild-build-esp --lib → 171 passed, 0 failed, 1 ignored
• soldr cargo clippy --workspace --all-targets -- -D warnings → exit 0
• soldr cargo fmt --all --check → exit 0

Not in scope (follow-up)

source_scanner::walk_sources walks the project src/ with the same
no-follow default, so a symlinked translation unit inside src/ is never
compiled. Same class of bug, different enumeration; left for its own change.

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