#205 follow-up: wire library-selection cache into orchestrators (teensy, stm32) #214

Context

Phase 4 of #205 (PR #212) shipped the resolve_cached(...) helper in crates/fbuild-library-select/src/cache.rs with a zccache_artifact::KvStore backend and 7 cache tests. The helper is correct in isolation but no production code calls it — framework_libs::resolve_framework_library_sources (the orchestrator entry point) still calls the uncached resolve(...).

Result: every build re-runs the LDF walk from scratch even though the resolver output is a pure function and the cache infrastructure is in place. This is the prerequisite for Phase 7 warm-cache benches (bench/fastled-examples) — there is no warm path to measure today.

Scope

1. Add a cached entry point in crates/fbuild-build/src/framework_libs.rs (e.g. resolve_framework_library_sources_cached) that takes the same (libraries, project_dir, src_dir) plus CacheKeyInputs and a KvStore.
2. Open a process-shared KvStore rooted at ~/.fbuild/cache/library-selection/ (or the daemon equivalent under dev/prod paths via fbuild-paths).
3. Update crates/fbuild-build/src/teensy/orchestrator.rs:163 and crates/fbuild-build/src/stm32/orchestrator.rs:114 to call the cached variant. Construct CacheKeyInputs from the orchestrator's Framework (install path, version) and the platform's compiler triple.
4. Refresh docs/architecture/library-selection.md status block (Phase 4 has shipped; the gating note pointing at zccache#130 is stale).
5. Add a test in framework_libs.rs that round-trips through the cached variant and asserts a second call hits.

Out of scope

• ESP32 / AVR orchestrators — they don't call resolve_framework_library_sources today; tracked separately if needed.
• Cache eviction / LRU — zccache_artifact::KvStore handles its own retention.
• Phase 7 warm bench harness (separate follow-up).

Acceptance

• All existing 16 library-select tests + 4 framework_libs tests still pass.
• New cache-hit test passes.
• cargo doc --no-deps -p fbuild-build clean.
• Workspace clippy -D warnings clean.

Refs: #205 Phase 7 prep; closes Phase 4 production wiring.