feat(bloat): add fbuild bloat-diff <a> <b> subcommand

[zackees]

Summary

Closes#440 (Phase 4 of #434). Stacked on top of #445 → #443 → #436. Merge those first.

Subsumes the ad-hoc .claude/symbolaudit/diff.py the FastLED #2773 audit carried. Each input may be a report.json, an ELF, or a project directory — same resolution as fbuild bloat.

• Symbol identity:(demangled, archive, object). Two symbols with the same name in different archives stay distinct (matches FastLED diff.py semantics).
• Categorises each row:added / removed / grew / shrunk.
• Default output dir:<project>/.fbuild/build/<env>/bloat-diff/<a-stem>__vs__<b-stem>/ when both inputs share a project, else <cwd>/bloat-diff/<a>__vs__<b>/. Writes both delta-report.json + delta-report.md.
• Region filter:--region flash (default) or --region ram. Net totals always cover both regions.
• --top N: Top-N movers in the MD table (default 25).

Refactor: shared analyzer entrypoint

Extracts bloat_cmd::load_or_analyze so both bloat and bloat-diff drive the same ToolPaths::resolve + analyze_elf pipeline. The helper also detects report.json inputs and short-circuits to serde_json::from_slice, so users can diff existing reports without re-running the analyzer.

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 — 41 passed (was 32 + 9 new)

New tests:

• diff_categorises_added_removed_grew_shrunk
• diff_net_totals_are_correct
• diff_filters_by_region
• diff_distinguishes_same_name_in_different_archives
• diff_rollup_by_archive_aggregates_net_change
• parse_region_defaults_to_flash
• format_diff_markdown_emits_expected_sections
• bloat_diff_command_parses_positional_args
• bloat_diff_region_flag_is_accepted

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