test(compat): expand independent real-world visual corpus

[akcorca]

Problem

The portable scorecard is strong but most committed PPTX fixtures are generated by wasmppt itself. Pixel and desktop-consumer evidence are also separate from the scorecard, and the visual manifest currently centers on one generated deck.

Scope

• Add independently produced, hash-pinned PPTX inputs to the browser visual corpus.
• Keep producer, version, license, redistribution, and expected diagnostic metadata explicit.
• Render declared slides through the real Wasm browser Worker and record per-presentation pixel evidence.
• Connect CI artifact generation without weakening existing structural, preservation, or security gates.
• Update compatibility, corpus, and rendering documentation.

Acceptance criteria

• At least two independently produced decks participate in a browser pixel smoke gate.
• Each case has deterministic bounds, non-empty scene assertions, stable diagnostics, and PNG/report evidence.
• Fetch-only fixtures remain hash verified and are not committed against their redistribution policy.
• The visual report distinguishes generated baseline checks from external smoke evidence.
• npm run precommit and the real browser integration pass.

[akcorca]

Implemented in the current working tree and ready for review. Added hash-pinned Apache POI WithMaster.pptx and bar-chart.pptx to a real scalar-Wasm Worker/Canvas smoke gate, PNG plus external-report.json evidence, CI fetch/render steps, manifest contract tests, and compatibility/rendering docs. Verified pixel hashes 5aab2722 and 96ae8c6f; npm run test:browser:external --workspace @corca-ai/wasmppt passes.