fbuild bloat: emit and preserve firmware.map automatically #491

Problem

fbuild bloat / fbuild symbols produces a useful per-symbol table from nm, but the archive / object / section columns come from the linker map (firmware.map). On a fresh fbuild build, no map file is emitted, so those columns are all - and bloat lookup cannot show callers, referencers, or per-archive rollups.

Repro on this branch (Windows, esp32s3 sketch):

fbuild build tests/platform/esp32s3 -e esp32s3
fbuild symbols tests/platform/esp32s3 --top 50
# → all ARCHIVE/OBJECT/SECTION columns are "-"
# → "Flash bytes by archive" rolls everything up under "(unattributed)"

find tests/platform/esp32s3/.fbuild -name '*.map'    # → no results
find tests/platform/esp32s3/.fbuild -name 'build_info*.json'  # → no results

The map is the difference between a generic top-N report and an actionable one (e.g. "12% of flash is libc.a/lib_a-vfprintf.o").

Proposed fix

When fbuild bloat (today: fbuild symbols) runs against a project directory, if no map sits next to the ELF:

1. Re-link with -Wl,-Map=firmware.map automatically (or pass it on every link by default during fbuild build — the cost is a few hundred KB of disk).
2. Likewise emit build_info.json (toolchain nm / c++filt paths) next to the ELF so --nm / --cppfilt don't have to be passed manually.

Option 1 (always emit during link) is cleaner and matches what PlatformIO / Arduino IDE do today — the map is a normal build product. Option 2 (lazy re-link on demand from fbuild bloat) avoids the disk cost but makes the first invocation slow and surprising.

Acceptance

• After fbuild build tests/platform/esp32s3 -e esp32s3, firmware.map and build_info.json exist beside firmware.elf.
• fbuild symbols tests/platform/esp32s3 fills the ARCHIVE / OBJECT / SECTION columns.
• "Flash bytes by archive" shows real archive names (libc.a, libfreertos.a, etc.), not (unattributed).
• fbuild bloat lookup <symbol> reports per-symbol callers / TU-level referencers.

Related

• Found while investigating ~26 KB of newlib printf bloat on an esp32s3 firmware. Without archive attribution the report didn't even confirm the printf symbols come from libc.a — had to infer from the _r suffix and _dtoa_r signature.