v19: move storage and substrate compatibility to explicit subpaths

[flyingrobots]

Part of #712.

Problem

Root currently exposes adapter and substrate names that make application code think in graph persistence. v19 needs explicit subpaths for supported storage, formal advanced concepts, diagnostics, and compatibility.

Target Subpaths

@git-stunts/git-warp/storage
@git-stunts/git-warp/advanced
@git-stunts/git-warp/diagnostics
@git-stunts/git-warp/legacy

Scope

• Move GitGraphAdapter style root imports to storage or legacy equivalents.
• Expose GitStorageAdapter and MemoryStorageAdapter from storage.
• Move graph diff/materialization inspection to diagnostics.
• Move old graph/patch/worldline symbols to legacy or advanced according to whether they are compatibility or formal expert API.
• Update package.json exports and generated reference inputs.

Acceptance

• New application code can import storage without graph vocabulary.
• Legacy graph-shaped imports are explicit and reviewable.
• advanced contains formal concepts, not a junk drawer of old root exports.
• Node, browser, and type entrypoints agree on the subpath map.

[coderabbitai]

🔗 Related PRs

#24 - feat: InMemoryGraphAdapter (MEM-ADAPTER, v10.7.0) [merged]
#93 - Fix recursive tree OID read fanout and release v17.0.1 [merged]
#107 - Prepare v18.0.0 release metadata and final gates [merged]
#654 - Clean up versioned runtime names [closed]
#660 - Replace static assertion batch 4 [merged]

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