perf(runtime): compile raw-text closing tags once in WebFetch

[E2ern1ty]

Summary

The HTML nesting scan built a fresh closing-tag RegExp for every script, style, textarea, and title element it met, so a script-heavy page recompiled one of four identical regexes per element. The raw-text element set is fixed, so the matchers are now derived once at module level.

Sharing a cached RegExp instance is safe here because the scan assigns lastIndex immediately before every exec, so no state leaks between uses.

Also adds a test for the raw-text skip path these lines implement, which was previously untested: tag-shaped strings inside script/style content must not count toward the nesting limit (600 fake <div>s inside a style rule would trip the depth limit if the scanner believed them).

Verification

• npm --workspace @maka/runtime run typecheck — clean
• npm --workspace @maka/runtime run test:dist — 3350 pass / 0 fail, including the new test
• npx biome check on both changed files — clean

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: ZCode authored the change, the new test, and this description.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No