fbuild bloat-diff: per-symbol delta between two reports (Phase 4 of #434) #440

Background

Phase 4 of #434. Depends on Phase 3 (#439).

The FastLED #2773 audit carried a custom .claude/symbolaudit/diff.py
for the delta between two report.json files (e.g. 3.10.3 vs master).
Subsume it into a first-class subcommand.

What to do

1. New fbuild bloat-diff <a> <b> subcommand

Inputs:

• Two paths (each may be a project dir, a report.json, or an ELF —
  same resolution as fbuild bloat).
• --top N (default 25).
• --region flash|ram (default flash).

When given an ELF or project dir, the diff command runs fbuild bloat on it first to materialise the JSON, then proceeds. Cached
reports already on disk are reused if their mtime is newer than the
ELF's.

2. Output shape

Same dual report.json + report.md shape as fbuild bloat, but
content is a delta table:

• Added symbols (present in B, absent in A).
• Removed symbols (present in A, absent in B).
• Grew symbols (size_B > size_A, same key).
• Shrunk symbols (size_B < size_A, same key).
• Per-archive rollup of net change.

Default output path: <cwd>/bloat-diff/<a-stem>__vs__<b-stem>/report.{json,md}.

3. Symbol identity

Match symbols by (name, source_archive, source_object) tuple. This is
what the existing FastLED Python diff.py does and what the audit
report relied on.

Acceptance

• fbuild bloat-diff <3.10.3-project> <master-project> reproduces the
  #2773 audit report verbatim (modulo timestamps).
• The diff command works against any combination of project / report /
  ELF inputs.
• The Markdown output renders a GitHub-friendly table.

Related

• Meta: fbuild bloat — first-class, end-to-end bloat analysis with build_info-driven toolchain resolution (no manual nm/c++filt, no PlatformIO, no side scripts) #434 (meta).
• fbuild bloat: default output dir under build tree + path printing on exit (Phase 3 of #434) #439 (Phase 3).
• FastLED #2773 — the audit that motivated diff.py.