feat(context): compact measured requests and isolate foreground usage

[Brian Krabach (bkrabach)]

Summary

Adds the optional context.measured_request_view capability to context-simple when token_meter: "actual" is enabled. It lets a compatible orchestrator count the complete provider-facing request and preserve the exact counted envelope for dispatch.

Behavior and compatibility

• Context provides a single notice-inclusive candidate snapshot; the orchestrator owns its overlays and the provider-count callback.
• Provider input counts drive the existing configured trigger and target across no more than the existing eight legal compaction rungs. Protected floors are reported honestly, and a known hard-limit oversize fails closed.
• Missing or malformed soft measurement safely rolls back to the original counted view or the legacy actual-meter path; staged sticky decisions use a cancellation-aware transaction and commit predicate immediately before dispatch.
• Required system, developer, human/reminder, protected-tool, and tool-pair boundaries remain protected. Compaction notices remain qualitative rather than presenting local estimates as provider token totals.
• Foreground-usage ownership is persistent: after the claimant records scoped foreground usage, the broad generic response hook remains observational, including between requests.
• The capability is optional and negotiated. Older callers and non-participating orchestrators retain their prior paths. No new user configuration, Core/dependency/SDK, or CI changes are included. Breaking changes: none.

Validation

• Offline exact-archive validation with released Core 1.6.1: Context suite 209 passed, 1 existing expected xfail; no live count or model calls.
• Focused coverage: 33 new measured-view cases and 19 existing hard-fit cases passed. Eleven shared R2 regression cases were confirmed failing before commit 36688f52c803958037b53715fddf1fca2e7c9ba1, without fixture errors.
• Exact-candidate coupled validation: 15 passed using Loop a9544aa70f16e3dca38596c56963fac2c072c7ec and Anthropic 421e4506d91b7814c9653f4376d3a24bfbf5b444 (10 measured-path, 2 strict-400, 3 finalization/import), with SDK-boundary mocks. The oracle was synthetic canonical-JSON UTF-8 projection bytes, not native-token-accuracy, live-provider, performance, or quality evidence.

Release sequence

Context may merge independently of the Anthropic producer. The Loop consumer follows only after both producers are merged and their exact main CI is green; its publication happens afterward. This PR includes no host rollout, merge, auto-merge, review bypass, or administrative action.