test(tenancy): prove application read isolation (#157)

[smiggleworth]

Summary

• Add two-populated-tenant standalone and split API/worker broker proof for application revision, system-instance, application-boundary-reference, and system-instance-boundary-reference lists and exact reads over HTTP and MCP. The fixture also checks populated current draft application change previews.
• Follow each limit=1 list to its terminal cursor with a finite page cap and require exactly the seeded tenant-local IDs and content markers. A cursor issued in tenant A fails immediately in B with HTTP BadRequest or MCP Validation. Foreign IDs, including an A instance beneath a valid B application, and an unrelated authenticated user receive NotFound.
• Update the [EN-01 backend] Deliver tenant authorization and actor attribution #157 central read inventory with merged test(tenancy): prove Program and service read isolation (#157) #370 Program/service, test(tenancy): prove boundary and snapshot read isolation (#157) #372 boundary/snapshot, and test(tenancy): prove identity read isolation (#157) #373 identity evidence, this application slice, and precise remaining limits. No production leak was found in the exercised paths. Advances [EN-01 backend] Deliver tenant authorization and actor attribution #157; the issue remains open.

Validation

• Client check — exact-head CI checked the client; no client source changed.
• Format check — dotnet format Compliance.slnx --verify-no-changes --no-restore passed on the final local tree, 7a5d6cc.
• Release build — the focused backend command compiled the Release application and tests on the final local tree with no reported errors or warnings.
• Focused broker tests — ./scripts/check-backend.sh focused 'FullyQualifiedName~ApplicationReadLeakMatrixE2ETests' passed standalone and split-host cases, 2/2, on the final local tree.
• Full broker and native AOT validation — run 35926548187 passed Validate (including full real-broker tests), dependency review, and native AMD64/ARM64 at 7a5d6cc0aa5f3b8c6ab62ef212bdce9a48988f12.

Contracts and operations

• Authentication and authorization effects considered — an actor belongs to two populated tenants; an unrelated authenticated actor and foreign record/parent IDs exercise non-disclosure. No policy changed.
• API, persistence, scheduling, and wire compatibility considered — test and documentation only; no runtime contract, event, projection, job, or schema change.
• Deployment, health, logging, and rollback effects described — no deployment or configuration change; reverting removes only evidence tests and documentation.