docs: SOM overview + ZMX importer pipeline + proto build notes

[MarkBedrock]

Summary

• SOM overview: units policy (diameters, preserved lens units), copy semantics (EC/backup), tuples.
• ZMX importer pipeline outline (tokenize → tables → SystemModel → Intended Path; stop-surface notes).
• proto/README with protoc instructions.

Why

Gives Bedrock/Phoenix stable data definitions and import paths without adding implementation code yet.

Files

• README.md
• docs/som-overview.md
• docs/importers/zmx-parser.md
• proto/README.md

Checks

• ✅ Build check should pass (docs-only)
• ℹ️ CodeQL present (not required)

Summary

• What does this change add or improve?

Type of change

• Docs (guides, tutorials, how-to)
• RFC / governance
• Website content
• Community process / templates
• Other (describe)

Related issue / discussion

• Links to an issue, discussion, or RFC (if applicable)

Checklist

• Clear titles and headings
• Links work and relative paths are correct
• Images (if any) have alt text and acceptable licensing
• No secrets or proprietary info
• License headers/attribution included where needed

AI Assist (provenance)

• AI used? [ ] No [ ] Yes — Tool: (Copilot/Claude/Other)
• If yes, briefly describe scope (e.g., outline, grammar, boilerplate)

Notes for reviewers

• Anything specific you want feedback on?