feat: interface registry for cross-IU contract consistency

[Steve]

Summary

• Adds an interface registry — a single source of truth for how each IU is exposed at the architecture level (mount paths, resource field descriptions)
• Eliminates mount path mismatches between IUs (e.g., web UI calling /todos when tasks are mounted at /tasks)
• Post-processes LLM output to deterministically repair any fetch paths that don't match the registry

Problem

Two IUs that need to interact (Web Experience consuming the Tasks API) were generated independently with no shared contract. The mount path was derived separately in the scaffold generator and the prompt builder using the same heuristic — but the LLM could ignore the prompt instruction entirely and invent its own paths. This caused 404s at runtime.

Approach

The architecture target computes the interface registry from the IU plan — this is an internal compilation artifact, not a conservation layer. Both the scaffold (server.ts mount paths) and the prompt builder read from the same registry. A post-processing step in regen.ts rewrites any fetch() paths that don't match the registry using stem/synonym matching (todo→task, etc.).

Resource field descriptions are extracted from canonical "has" statements (e.g., "a project has a name and a color") and included in the prompt so consumer IUs know the provider's data shape.

Changes

• src/scaffold.ts: New InterfaceEntry type, deriveInterfaces() function, extractResourceFields(). generateScaffold() uses registry for mount paths instead of inline derivation.
• src/llm/prompt.ts: buildPrompt() accepts InterfaceEntry[] instead of string[]. Includes resource shape in sibling module context.
• src/regen.ts: RegenContext carries interfaces. New repairFetchPaths() post-processor with resourceSimilarity() scoring.
• src/cli.ts: Both bootstrap and regen compute deriveInterfaces() once and pass to RegenContext + scaffold.

Test plan

• All 413 existing tests pass
• Bootstrap on examples/todo-app generates correct mount paths in server.ts
• Web UI fetch calls use /tasks and /projects (matching mount paths)
• Web UI includes project creation (POST /projects) — resource fields in prompt give the LLM enough context
• Fetch path repair correctly rewrites /todos → /tasks when LLM ignores instructions
• App runs and serves working UI at localhost:3000

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