Document native marker detection capability gap

[Serph91P]

Summary

• defines the strict public-SDK contract required for provenance-safe native marker contributions
• records why Emby 4.9.1.90 cannot invoke native intro or credits detection for one item with an exact run and marker receipt
• adds a fail-closed capability seam and focused regression tests
• deliberately adds no detector runner, capture, media access, preview, approval, persistence, UI, or submission path

Result

The current public Emby plugin SDK is unsupported for issue #28's provenance requirements. Metadata refresh, scheduled tasks, library scans, chapter snapshots, and intro-media provider APIs do not return an attribution-bound native detector run or exact generated markers.

The capability therefore reports all required flags as false. Later contribution work must remain disabled until a future documented public SDK satisfies the full contract.

Verification

• focused capability tests: 6 passed
• full tests: 32 passed
• Release build: 0 warnings, 0 errors
• vulnerable package audit: no findings
• Gitleaks history and working-tree scans: no findings
• whitespace check: clean
• independent review: no blocker, high, or medium findings

Safety

• no STRM path or target was opened
• no media detection was triggered
• no plugin artifact was deployed
• no upload or external submission path was added

Related to #28. This PR does not close the issue.

[Serph91P]

Review result

Reviewed the current rebased head e7ae025 against issue #28 and the public Emby plugin SDK.

Findings

• The original formatting issue in NativeMarkerDetectionCapabilityTests.cs was corrected.
• No remaining blocker, high, or medium logic or security finding was identified.
• The capability contract fails closed unless single-item targeting, exact completion, and exact generated-marker output are all available.
• No detector invocation, STRM or media access, preview, persistence, UI, credential handling, or submission path is introduced.
• The documented Emby 4.9.1.90 SDK gap is reproducible from the public package signatures.
• As an additional currency check, public SDK prerelease 4.10.0.24-beta2 was inspected through its XML and exported public methods. It still exposes no item-scoped native intro or credits detector with exact completion and exact generated-marker output.

Verification

• Full tests on current main: 33 passed, 0 failed, 0 skipped
• Release build: 0 errors, 0 warnings
• dotnet format --verify-no-changes: passed
• vulnerable package audit: no findings
• git diff --check: passed
• added-line security scan: no findings
• GitHub Actions build and test: passed on head e7ae025

Decision

PR #29 is complete for its explicitly limited Phase 01 feasibility-spike scope and can leave draft status.

Issue #28 is not implemented by this PR and must remain open with blocked: emby sdk. Its contribution workflow, preview, approval, receipt persistence, duplicate prevention, and submission acceptance criteria remain intentionally unimplemented until a suitable documented public Emby API exists.