Profile and shard core-and-rest compile path before the 240-minute cap

Finding

The current core-and-rest job reaches its 240-minute cap during compilation, before its relevant test workload is attributable. This is distinct from the excluded ruvector-delta-index deadlock tracked in #825 / PR #787.

• Severity: Medium functionality
• Operational impact: High — the required shard cannot provide regression signal
• Confidence: High for the observed compile-time bottleneck; root cause still unlocalized
• Estimated effort: 1–2 days
• Expected impact: restore a bounded, attributable Rust test gate and prevent unrelated PRs from inheriting a four-hour cancellation.

Confirmed evidence

In PR #926 Workspace run 32702332738, 16 of 17 jobs succeeded. Job 97356253593 was cancelled at the cap while Cargo/Rustc were still compiling ruvector-temporal-tensor-wasm. No DeltaHnsw test ran.

The current workflow explicitly excludes ruvector-delta-index from both core-platform and core-and-rest, so PR #787 cannot by itself explain or clear this current failure. The causal correction is recorded on #825.

Reproduction / measurement

1. Run the current core-and-rest job with timestamped Cargo build/test phases and per-package timing.
2. Record the last active Cargo/Rustc process at cancellation.
3. Separate dependency compilation, crate compilation, test compilation, and test execution time.
4. Repeat on a warm cache and a cold cache to avoid attributing cache behavior to test hangs.

Acceptance test

• Identify the package or dependency phase consuming the active compiler budget.
• Split or cache the shard so an attributable core-and-rest run completes below 240 minutes.
• Report compile time separately from test time.
• Run the focused delta-index suite independently under the timeout policy.
• One clean PR run completes with the shard's intended tests executed; no gate is weakened or removed.

This issue intentionally proposes no CI-policy patch; workflow changes require maintainer review.

Start by running the current core-and-rest job and timestamping Cargo/Rustc phases, including compilation of ruvector-temporal-tensor-wasm. Compare cold- and warm-cache runs, identify the package or dependency consuming the compiler budget, and run the focused delta-index suite independently. Done means one clean run finishes below 240 minutes with intended tests executed and compile time reported separately.