merge_with_server_issues runs whenever preflight validation produces issues, which with the documented oninput pattern means on every keystroke while a form has issues. Merging costs O(S·C + M·K·log M) for S server issues, C client issues, M merged issues and K form keys. 🤓 This change drops it to O(S + C + K + M·log M). 🤓

N fields with N issues (half server, half client), Node 22 x64, median of repeated runs:

Small forms are at parity or slightly faster, since the old code already allocated the key array.

Output is unchanged: identical issue-object ordering to the previous implementation across 10,000 randomized cases covering duplicated form keys, duplicated issue names, names absent from the form, mixed server and client issues and empty collections. The merge semantics negotiated in #14744/#14759 are preserved.

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