perf(process): reuse session receive cases

[wolfy-j]

Summary

Reuse the receive-case arrays and descriptors in the Session ingress and shutdown loops instead of rebuilding them on every select.

• Keep the ingress order: inbox, events, bus completion, policy requests.
• Keep the distinct shutdown order: bus completion, policy requests, inbox.
• Construct shutdown cases only if the bus completion has not already been consumed.
• Keep cases private to each run and let each select return a fresh result.

The channels are fixed throughout a run. Current master already waits directly on the command bus wake channel, so command_bus.lua is intentionally unchanged. No API, configuration, dependency or admission-policy change.

Verification

Test-first: on unchanged production code, the two reuse regressions failed while the other 334 cases passed. After the change, all 336 tests pass.

• Current Hub stable runtime v0.3.44a: 10 consecutive full-suite runs, 336/336 each; lint clean across 171 entries.
• Development runtime built from runtime #904: 10 consecutive full-suite runs, 336/336 each; lint clean across 171 entries.
• Independent Astra review approved; its own full-suite run passed 336/336.
• git diff --check passes.

The regressions check per-loop reuse and construction counts, distinct priority orders, invocation isolation, durable steering input, late-input rejection, and skipped unused shutdown cases. A real-channel test checks concurrent descriptor reuse, exactly-once delivery, independent retained results, and closure.

The scripted ingress tests use real SQLite admission/persistence but do not schedule the command bus or call an LLM. This establishes removal of repeated case construction, not a measured whole-Session latency or RAM improvement. It does not require runtime #904.