perf(build): no-change rebuild recompiles the entire sketch + core variant (~103s; should be seconds)

[zackees]

Context

#942 Phase-1 baseline (NightDriverStrip env:demo, 4 vCPU Linux container, median of 3): a no-change rebuild takes 107.8 s. Per-phase data shows compile-sketch=92.0s and compile-core-variant=10.7s re-run on every build even when zero files changed; only fw-libs short-circuits (0.001 s). fast-path-check=0 ms in every run — the fast path never engages for the sketch/core paths. The embedded zccache service also doesn't convert these recompiles into cache hits (hot compile-sketch ≈ warm compile-sketch).

For comparison, fw-libs demonstrates the desired behavior — its fingerprint check drops 150 s to 1 ms on hot rebuilds.

Proposal

Make the no-change rebuild converge to a fingerprint check + link-skip:

• Extend the fast-path/fingerprint coverage to sketch sources and the core variant (same mechanism fw-libs uses).
• Investigate why per-TU zccache lookups miss on identical recompiles (env/cwd/argv nondeterminism in the key?) — hits there would also accelerate the warm scenario.
• Skip link + elf2image when all inputs are unchanged and artifacts exist.

Acceptance criteria

• Hot (no-change) rebuild of NightDriverStrip demo drops from ~108 s to ≤ 10 s in the meta: profiling-driven build performance burndown (Docker Linux, NightDriverStrip, cold vs hot cache) #942 harness (stretch: ≤ 3 s).
• compile-sketch and compile-core-variant phases report ~0 ms on a no-change rebuild.
• Before/after harness numbers in the PR description.

Part of #942 (baseline: #942 (comment) — Phase 1 comment).