Add typed recursive WARP graph attachment references

[flyingrobots]

Plan: https://github.com/git-stunts/git-warp/blob/plan/streaming-recursive-materialization/docs/plans/streaming-indexed-recursive-warp.md
Design PR: #813

Outcome

Model a graph-valued attachment as a validated WARP graph reference rather than an undifferentiated PropValue or byte asset.

Required work

• Define runtime-backed inline, asset, and WARP graph attachment kinds.
• Define graph identity, repository/resolver authority, coordinate/frontier posture, handle kind/schema, capability/trust, and retention posture.
• Support pinned immutable roots and explicitly live locators.
• Resolve a live locator once per reading execution and record the pinned result.
• Keep child graph ordinals local to the child manifest.
• Reject ambiguous plain-object graph references at admission boundaries.

Acceptance

• Typed graph references round-trip deterministically.
• Pinned references participate in commitments without opening child state.
• Live references resolve once and receipts name the observed root.
• Missing capability, unsupported schema, and unresolved locator fail closed.
• Cross-repository resolvers do not leak storage mutation authority into the domain.
• Generic attachment docs distinguish assets from graph links.

[coderabbitai]

🔗 Related PRs

#732 - Fix: anchor state-cache payloads against Git GC [merged]
#749 - refactor(storage): delegate immutable payloads to git-cas [merged]
#751 - feat(materialize): add retained CAS materialization handles [merged]
#762 - Read retained properties through git-cas 6.5 [merged]
#813 - Define streaming indexed recursive WARP architecture [open]

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