Add targeted tree placement lookup

[marcus-pousette]

Summary

• add optional tree.placement(node) read API that returns a node's current parent plus restore placement
• implement the lookup in the shared SQLite adapter without reading all siblings
• expose it through sqlite-node and wa-sqlite RPC/direct clients
• make edits.capture use the targeted lookup when available, with the existing sibling-scan fallback for other backends
• add conformance coverage for first/after placement, moves, deletes, root, and missing nodes

Stack note

This is an exploratory draft stacked on #173. The public undo API remains edits.capture/undo/redo; this PR only tries a lower-cost backend read so move/delete capture does not need tree.children(parent) on SQLite-backed runtimes.

Verification

• pnpm -C packages/treecrdt-ts run build
• pnpm -C packages/treecrdt-engine-conformance run build
• pnpm -C packages/treecrdt-wa-sqlite run build:ts
• pnpm -C packages/treecrdt-sqlite-node run test

Known local limitation

pnpm -C packages/treecrdt-wa-sqlite run test still fails in the vendor build with No rule to make target tmp/obj/dist/sqlite3.o, before Vitest runs. The wa-sqlite TypeScript/RPC path compiles.

[marcus-pousette]

Closing this as superseded by #175. This was an optimization for eager capture reads; #175 defers inversion to undo time, so targeted placement lookup is not needed for the first clean solution. We can revisit it later as a separate optimization if eager capture remains useful.