Migrate retained substrates directly into indexed graph roots

[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

Replace the v18-to-v19 monolithic replay-basis translation with a script-owned streaming migration that constructs dictionaries, address pages, Roaring roots, causal roots, attachments, graph references, and a retained manifest directly.

Required work

• Keep all legacy decoding under scripts/.
• Integrate graph discovery, version status, one-pass confirmation, Bijou progress, capacity estimates, and multi-graph isolation from Make v18-to-v19 migration discover graphs before confirmation #806–Define and satisfy the v19 migration guide requirements #810.
• Stream legacy checkpoint components and writer patches through bounded reducers.
• Verify bounded reopen, append, property reads, attachment streaming, neighborhood and recursive traversal, receipts, and semantic equivalence in scratch.
• Refresh every source ref and promote through compare-and-swap with additive recovery refs.
• Re-run the same verification after promotion and roll back safely on failure.

Acceptance

• Migration never constructs or publishes a full-state replay basis.
• No production full-state codec is called.
• A generated legacy state exceeding 5 MiB migrates under a reviewed heap cap.
• Normal execution performs expensive migration work once.
• A disposable real Think copy passes before the authoritative graph is retried.
• Make v18-to-v19 migration discover graphs before confirmation #806–Define and satisfy the v19 migration guide requirements #810 are completed against the new representation.

[coderabbitai]

🔗 Related PRs

#107 - Prepare v18.0.0 release metadata and final gates [merged]
#732 - Fix: anchor state-cache payloads against Git GC [merged]
#771 - feat: resume retained materialization predecessors [merged]
#788 - Add one-shot v18-to-v19 retained-substrate migration [merged]
#813 - Define streaming indexed recursive WARP architecture [open]

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