GP6-S1: Add WAL-backed strand fork and drop intent receipts

[flyingrobots]

Parent: #604

User story

As Echo, I need strand topology changes to cross the same accepted-intent boundary as edits, so a forked writer universe can be recovered after restart without relying on process-local registry state.

Scope

• Define topology intent shape for strand fork and, if supported, strand drop.
• Return a StrandForkReceipt or successor receipt whose identity binds source worldline, fork tick, commit/boundary hash, child worldline, writer heads, retention posture, and issuer/session evidence.
• Add WAL record kinds or equivalent WAL payloads for accepted topology intent evidence.
• Keep application code unable to append WAL directly.

Acceptance criteria

• A strand fork cannot be acknowledged until its topology evidence is committed to WAL or an explicit unavailable obstruction is returned.
• Replayed WAL can rebuild enough fork evidence to restore strand identity and child worldline mapping.
• Failed WAL commit leaves no visible half-fork in runtime/provenance state.
• Tests cover success, duplicate/idempotent fork, WAL failure rollback, and recovery.

Non-goals

• No jedit/editor nouns in Echo core.
• No Git branch/worktree integration.

[coderabbitai]

🔗 Related PRs

#323 - Witnessed suffix admission evaluator [merged]
#329 - Stack Witness 0001 fixture-backed jedit walking skeleton [merged]
#339 - feat(core): add obstruction receipt skeleton [merged]
#379 - feat(core): commit runtime WAL ACK evidence [merged]
#598 - feat(warp-core): add filesystem WAL failure atomicity [merged]

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