perf: avoid quadratic remote form issue merging

[Nic-Polumeyv]

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:

fields	before	after	speedup
400	1.6 ms	0.1 ms	15×
800	7.9 ms	0.3 ms	27×
1600	33.0 ms	0.8 ms	42×
3200	163.8 ms	2.2 ms	76×

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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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 986b3e6:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/986b3e6b76b8a5660b94d4a1a7ac6424865632cc

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16493

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Latest commit: 986b3e6

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