Avoid cloning keyed comprehensions without moves

[preciz]

@SteffenDE I'm not familiar with the JS side of things but this seems like a small diff for a good gain in performance.

Assisted by: GPT 5.6 Sol - Codex

Clone keyed state only when a diff moves entries. Stable-position
updates never read another old position, so they can merge in place
instead of copying the full cached render tree.

Keyed entries are encoded as:

• kc: resulting entry count
• object: new entry or same-position diff
• number: old index for a move without a diff
• [old_index, diff]: move with a diff

Only the two move encodings read old positions and need a snapshot
before mutation. Nested keyed diffs apply the same rule, cloning only
the subtree that moves.

Chromium 150 merge-only timings (median of seven calibrated samples;
DOM patching excluded):

• 1,000/10,000 compact two-field rows: 0.435/4.29 ms -> 0.001 ms
• 1,000/10,000 rich rows (eight fields, four children): 3.94/47.5 ms -> 0.001 ms

Stable keyed merges inside LiveComponents dropped from two tree clones
to one, making merge-only benchmarks about 2x faster. Reordering
entries still needs the move-safety clone and showed no material change.