Preserve primary journal errors when failure-observation writes fail #853

Describe the bug
The production journal reader can replace an original Open/Copy/final-manifest failure with JournalObservationFailure when writing that failure's diagnostic also fails. The OpenFailed, CopyFailed and FinalManifestOpenFailed callsites invoke Write-CapJournalObservation directly before completing their primary-error handling.

This is the pre-existing, separately deferred risk identified during #851 / #848, still present after #852. It is distinct from the actor-cleanup message masking addressed by #852 R3.

To reproduce
Source-derived test recipe; a deterministic runtime reproduction of these three paths has not been executed for this issue:

1. Arrange an original journal Open, Copy, or final-manifest Open failure.
2. Make the corresponding observation sink unavailable, for example an observation path that cannot be opened/written.
3. Exercise each existing failure path. The observation helper can throw before the caller rethrows/classifies the original failure, making the secondary diagnostic failure the surfaced primary.

Add focused injected-sink tests for each path before implementing the correction. The existing deadline-observer test establishes preservation only for Assert-CapJournalTime, not these three callsites.

Expected behavior
Retain the original exception/native HRESULT and applicable typed journal classification as the primary cause, while surfacing the observation-write failure explicitly as a secondary diagnostic. Reuse the existing deadline path's preservation pattern where appropriate; do not silently swallow diagnostic errors or return success.

Keep the 1000ms budget, retry/attempt caps, absolute deadlines, terminal-only durability policy, success-observation counts and cleanup assertions unchanged. Scope this small follow-up to the three production failure paths, related tests and directly related documentation.

Screenshots
None. This is an offline journal/error-handling issue; no GUI capture is involved.

Environment

• GraphCode version: merged commit 77ec22136a99f55e7d0be00ce37979525753a57f from Keep journal retry observations buffered until terminal failure #852.
• macOS version: not applicable; source-derived Windows journal-reader risk, with no macOS reproduction or safety claim.
• Backend: offline Windows journal/file-I/O contracts; no paid backend or model interaction required.

Additional context
Relevant persisted callsites:

• Numbered Open failure.
• Copy failure.
• Final-manifest Open failure.

#852 deliberately limited primary-error preservation to the deadline-observation path and improved the separate test-harness actor-primary message. Its local/hosted green results do not establish injected-observer-failure behavior for these three production paths. No fix, new native run or budget change is assigned by filing this tracker.