M9-01: Keep stream-level writer honest

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L1018
Tracking commit: 1ec8105

Scope

Labels: priority:P1, type:test, area:writer

• Expand stream patch tests for:
  • grow
  • shrink
  • multi-stream
  • missing stream
  • corrupt gzip
  • unchanged identity patch
• Confirm GUID/history preservation behavior.

Acceptance criteria:

• Tests cover real family and project fixtures.
• Docs distinguish byte-preserving copy, stream patching, and semantic
  editing.

Definition of done

• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.
• Tests, fixtures, or documentation are added at the level appropriate to the change risk.
• User-facing behavior and limitations remain honest in README/docs/viewer messaging.
• Relevant CI checks pass before the issue is closed.

[DrunkOnJava]

Completed in 6044c12.

Implemented the stream-writer coverage expansion:

• Added missing-stream validation coverage that asserts StreamNotFound and no output file.
• Added corrupt-gzip verification coverage in writer unit tests.
• Added project-corpus patch coverage for identity, grow, shrink, and multi-stream patches while checking unpatched streams plus GUID/history preservation.
• Added a CI Writer patch corpus job that fetches both the family corpus and project corpus and runs cfb_roundtrip_delta.
• Updated writer/user docs to distinguish byte-preserving copy, stream patching, and unsupported semantic editing.

Verification:

• Local: cargo test writer::tests:: --lib, RVT_PROJECT_CORPUS_DIR=... cargo test --test cfb_roundtrip_delta -- --nocapture, cargo fmt --all -- --check, cargo clippy --all-targets -- -D warnings, workflow YAML parse, git diff --check, and stub-pattern scan passed.
• GitHub: Writer patch corpus job 73004021923 passed, and all standard check runs for 6044c12 are green.