test(compat): expand independent real-world visual corpus #82

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.