fix(build): bound Cargo target cache generations - #2025

Summary

• identify Cargo units from fingerprint metadata and retain the latest generation plus recently invoked variants
• prune stale fingerprint generations together with orphaned deps and build outputs
• scope compiler-busy detection to the selected target profile locks
• restore BITFUN_TARGET_GC_DRY_RUN behavior and document the 24-hour grace window

Root cause

The previous collector never removed fingerprint directories. Because deps artifacts were considered live whenever a matching fingerprint existed, historical deps generations accumulated indefinitely. The global cargo/rustc process check could also suppress collection whenever an unrelated worktree was compiling.

Validation

• node --test scripts/cargo-target-gc.test.mjs scripts/desktop-tauri-build.test.mjs
• pnpm run check:build-prereqs
• cargo check -p bitfun-desktop
• post-GC cargo check cache hit: 1.01 seconds
• debug and release dry-runs converged to no stale paths