Simplify rename pairing and index scoping

[ilyakuz-db]

Cleanup pass on top of #6138, kept separate so the changes are reviewable on their own. Behaviour-preserving: no golden files change, all 22 config-remote-sync acceptance directories pass unchanged.

Changes

Resolve the removed element once per rename candidate. sameElementApartFromKey, sameFieldsApartFromKey and multiBlockElement each re-ran resolveSelectors on the same remove path and re-did the same AsAny().(map[string]any) cast and withoutKey strip. They now take the already-stripped oldFields map, resolved once per remove in pairRenames. Drops three near-identical resolve-and-cast preambles and the O(removes x adds) re-resolution — a run renaming N elements went from ~3N tree walks to N.

scopedParent helper for index bookkeeping. scope + resolvedPath.Parent().String() was spelled out at four call sites in ResolveChanges plus adjustArrayIndex. Forgetting that prefix is exactly the cross-block index-shift bug the parent PR fixes, so it is now one named function rather than a string concat to remember.

blocksDefiningSequence: drop the O(n^2) dedup. The loop iterated sortedBlocks() (already distinct by map key) while re-checking !slices.Contains(blocks, block) on every matching location, so a block could never be appended twice anyway. Replaced with a single slices.ContainsFunc over the locations.

Tests

Ran the full config-remote-sync acceptance suite plus the package unit tests; ./task lint-q reports 0 Go issues.