[AUTH-001] Versioned declarative geometry and flow identity

[Hacks4Snacks]

Roadmap priority: P2

Goal: Let the review-friendly manifest reproduce pages, geometry, membership, and stable flow
identity while retaining deterministic defaults when optional fields are omitted.

• Define a versioned tmforge-manifest envelope and accept/migrate the current unversioned shape.
• Add pages and page identity; require flow endpoints to remain on one page.
• Add optional position and size for boundaries/elements plus explicit boundary membership.
• Add stable flow aliases and deterministic connector ids; resolve flows by alias in set/remove and
  preserve aliases through export/apply.
• Export and apply page, position, size, flow identity, and membership without moving explicitly
  placed objects.
• Validate that members fit their boundary, growing an auto-sized boundary or rejecting impossible
  explicit geometry instead of clipping.
• Keep geometry optional so concise manifests still receive deterministic placement.

Acceptance criteria:

• export -> apply -> export is stable for pages, aliases, geometry, membership, and flow identity.
• Existing unversioned manifests continue to apply or migrate with an explicit diagnostic.
• Canonical tmforge-json and .tm7 geometry remain unchanged by manifest evolution.

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