fix: renumber only the code blocks an edit changes

[Azganoth]

Summary

While code line numbers were shown, every document change rebuilt the line-number decorations of every code block in one new DecorationSet, which costs top-level blocks times decorations. Typing anywhere in a document with much code slowed with the square of its code, as highlighting did before #678.

• fix: highlight only the code blocks an edit changes #678's incremental update moves out of the highlight plugin into a shared codeBlockDecorations utility, and both plugins use it: the set is built once, then each transaction maps it and decorates again only the code blocks in its changed ranges, plus every code block inside a node whose own boundary it changed. Highlighting behaves as before.
• With line numbers shown, an edit outside code does no numbering work and an edit inside a block renumbers only that block. Turning the setting on still numbers every block, and turning it off clears them.
• docs/architecture.md describes the line numbers alongside the incremental highlighting, and the existing CHANGELOG.md entry for faster typing in code-heavy documents now covers line numbers.

Related Issue

Closes #679

Verification

• Focused tests compare the line-number decorations with a full rebuild after edits inside, between, and across code blocks, splits and joins, conversions between code and paragraph, wrapping in and lifting out of a quote, whole-document replacement, and toggling the setting with edits in between, and they assert which blocks are renumbered. Rebuilding the whole set, skipping the removal of stale decorations, or skipping the code blocks inside a node whose boundary changed each makes them fail. The highlight tests pass unchanged through the shared utility.
• pnpm check:frontend passed.

Manually verified in a happy-dom test mount on Windows 11, with a temporary probe that was not committed:

1. The fixture was about 1 MB of fenced code: roughly 1,030 30-line excerpts of the repository's TypeScript, Rust, and CSS, each after a one-sentence paragraph. Each keystroke's transaction was applied to the editor state seven times at the end of the middle paragraph and seven times inside the middle code block.

   Keystroke	main at fa9e842	This branch
   Paragraph, line numbers shown	163–476 ms	9–19 ms
   Code block, line numbers shown	158–182 ms	9–12 ms
   Either, line numbers hidden	6–24 ms	6–14 ms

Not verified: keystroke times in a release build.