Feat/custom component

[gitsad]

What does this PR do?

Adds the custom component type — a host-extensible escape hatch for components the built-in types
don't cover. Rather than a new type name per component, it's a stable envelope: name selects a
variant the host registered, props carries its inputs, actions wires its events. The spec stays
intent-level; all rendering lives in the host.

The key difference from the existing custom-type path (type: timer + customSchemas) is that the
envelope is LLM-authorable. A model can't invent type: timer — it isn't in the spec any prompt
teaches — but it can emit type: custom with name: timer once the variant appears in the catalog
the host passes to buildSystemPrompt({ customComponents }). Both paths share the same
customSchemas map, so a schema written for one is reusable by the other.

Also fixes a latent bug found along the way: the markdown pipeline was missing remark-gfm, so the
GFM tables models routinely emit next to charts silently degraded into run-on pipe text.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing behavior to change)
• Refactor (no functional change)
• Documentation
• CI / tooling

Additive only — existing documents parse identically.

Packages Affected

• @mobile-reality/mdma-spec
• @mobile-reality/mdma-parser
• @mobile-reality/mdma-runtime
• @mobile-reality/mdma-attachables-core
• @mobile-reality/mdma-renderer-react
• @mobile-reality/mdma-prompt-pack
• Blueprint: ___________

Also touched (no version bump — tests/docs only): mdma-mcp, mdma-validator, demo, evals.

Checklist

• I have read the CONTRIBUTING guide.
• 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) — 24/24 tasks.
• Type-checking passes (pnpm typecheck) — 22/22 tasks.
• I have added a changeset (pnpm changeset) — minor for the five packages above.
• New or changed MDMA schemas are backwards-compatible.
• Sensitive fields are marked with sensitive: true where appropriate — N/A, no new fields.

How to Test

1. pnpm install && pnpm test && pnpm typecheck — all green.
2. Envelope + per-name props validation: pnpm --filter @mobile-reality/mdma-parser test.
   Covers a registered name (props validated, errors prefixed props.*) and an unregistered one
   (passes through rather than failing the parse, so a document authored against a richer host still
   renders elsewhere).
3. 3D showcase: pnpm --filter @mobile-reality/mdma-demo dev → Docs → Components → custom.
   A WebGL 3D bar chart rendered through the envelope via three.js; drag to orbit, click a bar and
   confirm the selection lands in the action log.
4. Agent authoring it end-to-end: demo → Agent Chat → ▶ auto-play. The scripted run now closes on
   pls generate sample 3D graph; the agent should emit a type: custom / name: graph-3d block
   with its onSelect action wired.
5. Evals (needs API keys): pnpm --filter @mobile-reality/mdma-evals eval (33 cases incl. 5
   custom scenarios), eval:fixer (16), eval:guidance-coexistence (8).

Screenshots / Examples

type: custom
id: revenue-3d
name: graph-3d
label: "Revenue by Region and Quarter"
props:
  title: "Quarterly Revenue (3D)"
  bars:
    - { x: "EMEA", z: "Q1", y: 42 }
    - { x: "EMEA", z: "Q2", y: 58 }
actions:
  onSelect: bar-selected