docs: deepen vision and architecture into governing documents

[sandover]

Transform both docs from executive summaries into governing documents
that serve two functions: auditable quality criteria and hard invariants
that must survive evolution.

vision.md:

• Product principles now state violation conditions, not just intent
• "Local-first by default" → "Local-first" with explicit violation: any
  feature making network calls part of the default path
• Out-of-scope items promoted from positioning to hard stops with rationale
• Success criteria made checkable (observable, not aspirational)
• Strategy implications extended: principles win over features

architecture.md:

• Design principles now operational: each names what a violation looks like
• Layer model gains must/must not rules per layer, not just descriptions
• Cross-layer rule elevated and stated as the most important rule
• Write path gains invariants: lock scope, Writing-state frames, plan.rs purity
• Tail path gains invariant: correctness must not depend on notify delivery
• Transport architecture gains invariant: new transports must not require
  core changes
• Extension seams: "should" replaced with "must/must not" throughout
• New Quality criteria section: auditable claims checkable by inspection or test
• Data model: format version invariants added

https://claude.ai/code/session_01TB3GQER1hKkTxuYEAnHL4C