fbuild bloat: default output dir under build tree + path printing on exit (Phase 3 of #434) #439

Background

Phase 3 of #434. Depends on Phase 2 (#438).

Today fbuild bloat (formerly symbols) writes to wherever the user
points it with --json / --output-dir. The audit trail in FastLED
#2773 had users hand-typing those paths on every invocation. The TL;DR
example from #434:

$ fbuild bloat .
Building esp32s3 (release)...
Analyzing 4,591 symbols (nm via /home/.../xtensa-esp32s3-elf-nm)...
Demangling via /home/.../xtensa-esp32s3-elf-c++filt...

Bloat report:
  Flash: 388,380 B across 3,685 symbols
  RAM:    37,460 B across   906 symbols

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

What to do

1. Default output directory

When neither --json nor --output-dir is given, default to:

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

Always write both report.json and report.md.

Project + env come from the same build_info.json Phase 1 wired in
(via the env field + the build_info file's containing directory).
For ELF-only inputs (no build_info nearby), fall back to writing into
the ELF's parent directory under bloat-report/.

2. Print absolute paths on exit

Always print both written paths verbatim on stdout, one per line, even
when other output was streamed before. Acceptance:

fbuild bloat . 2>&1 | grep -E \"report\.(json|md)\"
# emits exactly two lines, both absolute paths under .fbuild/build/<env>/bloat-report/

3. Wire fbuild build --bloat (alias of legacy --symbol-analysis)

The existing --symbol-analysis flag morphs into --bloat. Keep the
legacy spelling as a hidden alias.

Acceptance

• fbuild bloat . with no flags writes both files to the default
  directory and prints absolute paths.
• fbuild build --bloat . runs the post-link bloat report.
• --symbol-analysis keeps working as a hidden alias.

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: rename 'symbols' subcommand + drop flag dependencies (Phase 2 of #434) #438 (Phase 2).