Feat/file upload input

[gitsad]

What does this PR do?

Adds a file field type for MDMA forms — forms can now declare file upload inputs alongside text/email/number/etc. Includes a default file input UI in the React renderer (overridable via ElementOverridesContext), spec schema + validator defaults, and authoring guidance in the prompt pack so LLMs know to use it.

While in the same area, also hardens the fixer prompt (Duplicate ID must rename instead of delete; LLM must not invent thinking blocks as commentary) and reorganizes fixer evals — multi-step flow tests now live in tests-fixer-flow.yaml with variantKey: flow, and flow-ordering exclusion is conditional on the variant. eval:fixer 10/10, eval:fixer-flow 7/7.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation

Packages Affected

• @mobile-reality/mdma-spec
• @mobile-reality/mdma-renderer-react
• @mobile-reality/mdma-runtime
• @mobile-reality/mdma-prompt-pack

Also affects @mobile-reality/mdma-validator (schema defaults for the new file field type) — not listed in the template above.

Checklist

• My code follows the existing code style (pnpm format and pnpm lint pass).
• I have added or updated tests that cover my changes.
• All tests pass (pnpm test).
• Type-checking passes (pnpm typecheck).
• I have added a changeset (pnpm changeset) if this change affects published packages.
• New or changed MDMA schemas are backwards-compatible (or marked as breaking).