feat(core): fold compactions into model requests

[LukasParke]

What

• fold compaction records before model request assembly in raw-log index space
• apply the same folding to preview requests
• emit {name}:context_pressure when folded history crosses compactAt
• emit at most once per step execution, without latching filtered/under-threshold checks

Why

Item 15 provides explicit compaction records and pure projection. The runtime must fold those records before system/history partitioning or replacesUntil indexes drift and live turns can be deleted. Pressure events let callers compact before hard budget trimming silently drops history.

This ports item 16 from fork/port/openrouter-fixes commits 1761e6b7 and b83bdd91.

Test plan

• 35 focused compaction/runtime tests pass
• core typecheck
• root lint
• sentrux check .
• sentrux gate .
• clean-context Pi review; added no-layer pressure emission and verified fold-before-partition semantics

Dependency

Depends on #80. The diff currently includes its prerequisite and must be rebased through main after #80 merges. Allocator changes are not included.

[vercel]

@LukasParke is attempting to deploy a commit to the Matt Apperson's projects Team on Vercel.

A member of the Team first needs to authorize it.

[LukasParke]

Agent: Code CI/structural/DCO are green where configured; remaining external red checks are compat with an empty upstream OPENROUTER_API_KEY and Vercel authorization. Where core CI is red, the only failure is the unrelated timing-sensitive executeRunCode retry-backoff test; affected local/full suites passed. I cannot rerun upstream Actions without admin rights.