Add capability registry, pipeline-driven adapters, and expanded format matrix

[gamerg21]

Motivation

• Introduce a capability-driven conversion substrate to make format support explicit and pipeline-aware for phased expansion beyond images.
• Prepare for multiple external tool adapters (FFmpeg, LibreOffice, Pandoc, Calibre, archive tooling) while keeping current image conversions stable.
• Expand the shared format matrix to cover audio, video, document, spreadsheet, presentation, ebook, and archive categories.

Description

• Add a capability registry (packages/conversion-engine/src/capabilities.ts) that maps source/target pairs to a pipeline (sharp|ffmpeg|libreoffice|pandoc|calibre|archive) and export findCapability and capabilityRegistry.
• Introduce ExternalToolAdapter (packages/conversion-engine/src/adapters/externalToolAdapter.ts) as a placeholder adapter that reports planned but unconfigured pipelines and displayNames for nicer errors.
• Update BasicAdapter to consult the capability registry via findCapability, tighten Sharp output checks, surface an errorCode on failures, and standardize metadata/pipeline reporting.
• Wire multiple adapters into the engine (packages/conversion-engine/src/index.ts), add findAdapter, and make executeConversion validate capabilities and return UNSUPPORTED_PAIR or TOOLING_UNAVAILABLE with contextual messages.
• Expand and densify the shared formatMatrix (packages/shared/src/formats.ts) to include broad audio, video, document, spreadsheet, presentation, ebook, and archive formats and add getCategoryForInput.
• Introduce typed error codes in types.ts and export new capabilities from the engine package, and add a new design doc docs/format-expansion-plan.md describing the roadmap and phases.

Testing

• Ran node:test unit tests in packages/conversion-engine and packages/shared, including index.test.ts and formats.test.ts, which validate adapter discovery (findAdapter('png','jpg')), capability resolution (findCapability('mp4','mp3')), tooling-unavailable behavior for planned adapters, unsupported-pair rejection, and new format matrix assertions; all tests passed.
• Adapter-level assertions confirm TOOLING_UNAVAILABLE is returned for planned non-image adapters and UNSUPPORTED_PAIR is returned when no capability exists.

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Codex Task