refactor(undo): iterate affected files child-first via recursive CTE

[mfreed]

QueryUndoAffectedFiles previously returned affected files ordered by file_id ASC, which approximates parent-first iteration (UUIDv7 = creation time, dirs are created before their contents). This ordering was the precondition for the cascade-artifact bug fixed previously: under parent-first iteration, each child UPSERT's bump_parent_mtime cascade landed on the already-restored parent and wrote a fresh history row -- newer than the parent's own restore row. The earlier post-hoc "newest history row" capture in Step 3 then resolved to the cascade artifact instead of the parent's own restore snapshot, silently corrupting undo-of-undo for directory renames.

The inline version_id capture made correctness order-independent, so this rewrite is defense in depth, not a correctness fix. Switch the affected-files query to a recursive CTE that walks each affected file's parent_id chain UP and sorts by depth DESC (deepest first, file_id ASC tiebreaker among siblings). Two-tier ancestor lookup: for ancestors in the affected set, use the parent_id from the entry's snapshot (affected.version_id -> history row); for untouched ancestors, use current source.parent_id. The snapshot path is what makes child-first ordering work for the rm-rf-then-undo case, where all affected source rows are absent and a source-only walk would default everything to depth 0.

Result: cascades from child restores either land on absent parents (cascade UPDATE matches zero rows, no row written) or on pre-restore parents (cascade row written but superseded by the parent's own subsequent UPSERT as newest). The parent's own restore row is always "newest" for the parent's file_id at end-of-transaction. If the inline version_id capture ever regresses, child-first iteration preserves the correctness invariant. Cascade-noise reduction in the common rm-rf-then-undo pattern is a bonus.

Function comment in query.go has the full design rationale, including the simpler form this rewrite replaces (kept inline as a reference).

Adds TestQueryUndoAffectedFiles_TopologicalOrder (unit), and TestSynth_UndoIterationOrder_ChildBeforeParent and TestSynth_UndoChildFirst_MinimizesCascadeArtifacts (integration). Cascade-noise test asserts delta=6 history rows added to d during rm-rf-then-undo; pre-rewrite the same scenario adds 9 (extra 3 cascade rows from Step-2 child UPSERTs onto the just-restored parent).