feat(generator): render the submittal register as a DOCX deliverable

[thewrz]

Why

The submittal register endpoint compiles products × required submittal types × datasheet
associations into structured JSON with findings — but the register's real-world destination is a
document deliverable issued alongside a package. Today every client that needs that document must
rebuild document rendering on its own, while SpecR's generator already owns DOCX output and CSI
styling. Rendering the register server-side keeps "one source of truth, many renders" true for
reports, not just specs.

What

• Generator module: SubmittalRegister → DOCX — a titled table document (grouped by section:
  product, required submittal types, datasheet references), with an optional findings appendix.
• Surface: format=docx content negotiation on the existing register route, or a sibling
  /submittal-register/generate — follow whichever pattern /specs/{id}/generate established;
  decide in review.
• Reuse generator infrastructure (dolanmiu/docx, existing style plumbing); no new styling system.
• Markdown render of the register via the shared renderer contract if cheap — keeps parity with
  how other artifacts render.
• MCP: follow the existing generate_docx tool pattern (or MCP_UNEXPOSED with rationale if
  binary payload doesn't fit the tool surface); contract-map.ts entry either way.

Tasks

• src/generator/: register→DOCX module (table layout, findings appendix)
• src/api/: route/param + openapi.yaml same PR
• src/mcp/: tool or explicit MCP_UNEXPOSED entry
• Tests: golden structural assertions (headings, row counts, grouping) — structure, not pixels
• Integration test: end-to-end from seeded project → non-empty DOCX

Acceptance criteria

• One call produces a well-formed DOCX register for a project (optionally scoped to a package),
  grouped by section, consistent with the JSON endpoint's rows.
• JSON and DOCX renders never disagree on content (same query underneath).