feat(server): prepare CPU placement for rebuild integration

[Mjones13]

The CPU placement work was prepared in the older t3code fork, while active rebuild work now belongs in Jones-Code. This setup checkpoint ports the placement helper, validation/bootstrap contracts and Effect spawner wrapper to the rebuild staging stack, with an explicit native build command and tests.

Targets work/jones-rebuild/staging; prepared and tested from 0ad04f19903fc5d9fe760620f3d19c27c93a52eb. Current staging advanced to c1ef5f06888753b9a11f8c1963ef54d9bc9aaaef through the peer-block layer; its changed paths do not overlap this setup. This is a draft integration checkpoint, not a merge-ready claim against the combined rebuild.

Runtime wiring remains with the owning takeover layers: launcher, provider/ClaudeV2, PTY, process attribution and CPU-aware queues. Release packaging, delegated cgroups and positive live placement are also deferred. These modules are dormant; environment configuration alone does not isolate the running app. The boundary documentation records the constraints for blending this with the other work. Supersedes source setup in t3code #21, rather than transplanting obsolete legacy launch hooks.

Validation: 5 native lifecycle tests and 47 placement tests passed, zero skipped; native Linux helper compilation, server Effect-aware typecheck, formatting and diff checks passed. Targeted lint exited 0 with five warnings. Scoped Knip exited 1 for the intentionally dormant PROCESS_PLACEMENT_ENV export and an unlisted cc binary in an untouched process-tree test; neither was suppressed. No host service, cgroup, installed runtime or UI was changed.

Implemented with GPT-6 Astra/high; coordinated with GPT-6.1 Sol through the Codex harness in T3 Code.

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