perf(sqlite): add placement order_key UDF (fixes #66)

[marcus-pousette]

Fixes #66.

• Adds treecrdt_allocate_order_key(parent, placement, after, exclude, seed) to the SQLite extension.
• Moves sibling-boundary lookup + allocate_between into the extension (single SQL call).
• Updates @treecrdt/interface SQLite local writer to call the UDF (removes boundary SQL + TS placement logic).
• Adds extension tests covering first/last/after/exclude + TRASH.

Notes:

• The UDF calls treecrdt_ensure_materialized() internally before reading tree_nodes.
• Callers should pass placement as first|last|after and pass after only for after placement; pass exclude to ignore a moving node.