feat(symbols): ingest toolchain paths from build_info.json

[zackees]

Summary

Closes #428 (Phase 1 of #434).

• Extends BuildInfo with nm_path / cppfilt_path / readelf_path / objdump_path plus a PIO-shape aliases block that mirrors them onto short keys (nm, c++filt, etc.). PlatformIO consumers (FastLED ci/util/symbol_analysis.py, ci/inspect_binary.py) keep working drop-in against fbuild-built artifacts.
• Derives the four paths centrally in BuildInfo::new from size_path using the GCC cross-tool naming convention (<prefix>-size → <prefix>-{nm,c++filt,readelf,objdump}). All of fbuild's existing orchestrators (AVR, ESP32, RP2040, STM32, Teensy, ESP8266, NRF52, SAM, Renesas, Apollo3, LPC, Silabs, CH32V) ship GCC cross-toolchains, so no orchestrator surgery is needed.
• Teaches fbuild symbols to auto-locate build_info.json by walking up from the ELF directory; adds a --build-info <path> explicit override. New resolution order:
  1. --nm / --cppfilt CLI flags (user wins).
  2. --build-info <path> if provided.
  3. Auto-discovery: walk up from ELF dir for build_info.json or build_info_<env>.json.
  4. PATH lookup of bare nm, with c++filt derived by stem.
  5. Hard error with a message pointing at all four sources.
• Common case becomes flag-less:

  fbuild symbols .fbuild/build/uno/firmware.elf
  fbuild symbols .pio/build/esp32s3/firmware.elf
  

• Adds docs/symbols.md walking through the resolution order, the aliases schema, and PIO/fbuild interop.

Test plan

• soldr cargo check --workspace --all-targets ✅
• soldr cargo clippy -p fbuild-build -p fbuild-cli --all-targets -- -D warnings ✅
• soldr cargo test -p fbuild-build --lib — 627 passed, 0 failed
• soldr cargo test -p fbuild-cli --bin fbuild — 25 passed, 0 failed
• New tests:
  • derive_gcc_tool_path_avr_prefix / xtensa_with_exe_suffix / bare_size_falls_back — derivation rules
  • build_info_populates_new_toolchain_fields — the four new fields land
  • build_info_aliases_block_mirrors_paths — PIO-shape mirror
  • build_info_aliases_omit_empty_optional_tools — ESP8266 (no ar/objcopy) keeps consumers safe with key in aliases
  • emit_writes_aliases_into_json — round-trip via JSON
  • find_build_info_walks_up_from_elf_dir / prefers_env_specific / returns_none_when_absent — discovery walker
  • load_build_info_unwraps_single_env — single-env outer object contract
  • resolve_reads_nm_from_build_info_auto_discovery — CLI resolver: build_info auto-pickup
  • resolve_explicit_nm_wins_over_build_info — CLI resolver: explicit override precedence
  • resolve_explicit_build_info_path_is_honoured — CLI resolver: --build-info flag

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 1a97d28c-7b86-4989-9acf-78b0599c035c

📥 Commits

Reviewing files that changed from the base of the PR and between 619d6ca and e284f3f.

📒 Files selected for processing (9)

• crates/fbuild-build/src/build_info.rs
• crates/fbuild-build/src/symbol_analyzer.rs
• crates/fbuild-cli/src/cli/args.rs
• crates/fbuild-cli/src/cli/dispatch.rs
• crates/fbuild-cli/src/cli/symbols_cmd.rs
• crates/fbuild-core/src/symbol_analysis/mod.rs
• crates/fbuild-core/src/symbol_analysis/tests.rs
• docs/CLAUDE.md
• docs/symbols.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/issue-428-buildinfo-toolchain-paths

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