fbuild build: default-on bloat report after link (Phase 5 of #434)

[zackees]

Background

Phase 5 of #434. Depends on Phase 3 (#439).

Same way fbuild build emits build_info.json by default today, make
it emit bloat-report/{report.json,report.md} by default post-link.

What to do

1. Default-on bloat report

After every successful link, run the bloat analyzer and drop:

<project>/.fbuild/build/<env>/bloat-report/report.json
<project>/.fbuild/build/<env>/bloat-report/report.md

No flag needed.

2. Opt-out flag

Add --no-bloat-report for CI matrices where bloat reports would blow
up the artifact size (~50–500 KB per env per build).

3. Print the paths on link exit

Same shape as Phase 3's bloat-on-stdout: fbuild build . ends with

Bloat report:
  /abs/path/.fbuild/build/esp32s3/bloat-report/report.json
  /abs/path/.fbuild/build/esp32s3/bloat-report/report.md

so users have the paths in their terminal without having to remember
them.

Acceptance

• fbuild build . on an esp32s3 Blink project drops
  bloat-report/report.json + report.md alongside the ELF without
  any extra flag.
• fbuild build --no-bloat-report . skips the report.
• Adds ≤ 1 s to incremental builds (analyzer is fast; symbol-extraction
  is the same nm call that already runs for size_path reporting).

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).
• fbuild bloat: rename 'symbols' subcommand + drop flag dependencies (Phase 2 of #434) #438 (Phase 2 — the bloat rename lands first).

[zackees]

Shipped via merged PR #447.