feat(build): default-on post-link bloat report

[zackees]

Summary

Closes#441 (Phase 5 of #434). Stacked on top of #446 → #445 → #443 → #436. Merge those first.

fbuild build now runs the fine-grained bloat analyzer automatically after a successful link and writes both report.json and report.md to <project>/.fbuild/build/<env>/bloat-report/. Pass --no-bloat-report to skip — useful in CI matrices where the extra artifacts would blow up download size.

Behaviour

• Default fbuild build . → emits bloat-report/{report.json,report.md} + the bloat summary on stdout.
• fbuild build --no-bloat-report . → skips the post-link analyzer.
• fbuild build --bloat <path> . → respects the explicit path (legacy daemon-side spelling, fbuild bloat: default output dir under build tree + path printing on exit (Phase 3 of #434) #439); does not double-up.
• fbuild build -t compiledb . → no ELF produced, no bloat report.
• Bloat-analyzer failures are non-fatal — the build itself already succeeded; we don't want a missing nm to flip a green build red.

Implementation

CLI-side hook in run_build: after the daemon returns success, call bloat_cmd::run_bloat against the project dir with all-defaults. Reuses the full toolchain auto-resolution from #428 + #438 + #439, so zero extra wiring per platform.

Test plan

• soldr cargo check -p fbuild-cli --all-targets ✅
• soldr cargo clippy -p fbuild-cli --all-targets -- -D warnings ✅
• soldr cargo test -p fbuild-cli --bin fbuild — 43 passed (was 41 + 2 new)

New tests:

• build_no_bloat_report_defaults_to_false — flag defaults match the documented behaviour
• build_no_bloat_report_flag_is_accepted — opt-out parses

🤖 Generated with Claude Code

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• Commit unit tests in branch feat/issue-441-auto-bloat

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