perf(build): framework libs (~150s) recompiled per project; core cache never hydrates

[zackees]

Context

#942 Phase-1 baseline, warm scenario (global cache fully warm, fresh project copy): fw-libs=150.7s re-runs in full for every new project directory even though the framework libraries are identical for a given platform + flag set. The fw-libs fingerprint only short-circuits within the same project (hot = 1 ms).

Related: core-cache-hydrate=0 ms in all 9 baseline runs while compile-core-variant always spends ~11 s and core-cache-store runs afterwards every time — i.e. the global core cache is written but apparently never hit, even cold → warm → hot inside one container session.

Proposal

• Scope the framework-lib artifact cache globally (keyed by platform/toolchain/flag fingerprint) so a fresh project hydrates ~150 s of compiled framework objects, mirroring the package disk cache design.
• Diagnose and fix the core-variant cache: why store-then-never-hydrate? (key mismatch across builds is the likely culprit.)

Acceptance criteria

• Warm scenario (fresh project, warm cache) fw-libs drops from ~150 s to ≤ 5 s in the meta: profiling-driven build performance burndown (Docker Linux, NightDriverStrip, cold vs hot cache) #942 harness.
• core-cache-hydrate actually hits on the second build with identical config (and compile-core-variant ≈ 0 s).
• Warm total drops from ~435 s toward ~compile-sketch + link.
• Before/after harness numbers in the PR description.

Part of #942.