Expose declared stop categories for missing account snapshot investigation

[Pigbibi]

The account-facts read-only diagnostic now exposes a fixed report_stop_reason from the same already-read report. This distinguishes declared skip, gateway, snapshot, permission and deadline categories when investigating missing account facts, without copying private error text or inferring a producer cause from an absent snapshot.

Classification requires the existing envelope, provenance and scope predicates. Conflicting or unknown categories remain fixed invalid/unclassified/unknown states. Original account stages, snapshot classification, protected revision expectation, publisher, report selection, CLI gate, workflow and cloud reads are preserved; no broker operation, production binding or dependency pin changes.

Validation: 213 focused cases (all original 190 plus 23 new); full suite 1,796 passed with 5 existing skips and 5 warnings; frozen sync, Ruff, lock/startup, all-workflow actionlint with ShellCheck and diff passed. Pin consistency passes using the repository's exact existing CI expectation. The separate default check against the public canonical QPK pin mismatches the retained runtime pin; that result is preserved and does not authorize adoption or deployment. The three-file source is frozen with the complete original test prefix preserved. Independent review completed with no actionable findings. Exact head f7ded3e51766bbb5a8481f2f792b238e44c366b1 PR CI 38447985496 and normal-merge e2ec696a9ba3ec64156804a701a556ad69d5b64b main CI 38448307666 succeeded; reviewed and merged trees match. Authorized existing main read-only run 38448514608 observed no_declared_stop in all four opaque slots, but all still failed the unchanged runtime_revision guard. Two slots had account_facts_absent and two structurally verified snapshots retained positions unknown; no publication occurred. These observations do not prove the source of the missing facts or serving approval. Evidence is recorded in QuantRuntimeSettings #632/#634.