fix: quadratic code-block scan in isInsideCodeBlock

[Vikramb1]

Problem

isInsideCodeBlock(text, position) rescans the entire prefix [0, position) on every call. Several handlers probe it once per candidate character — the link/image handler calls it for every [ while walking backwards — so total work is O(brackets × length).

The pathological case is exactly what streaming AI produces: a long, still-unclosed code fence full of brackets, re-repaired on every arriving token. Repro (58k chars):

const line = "const x = arr[i]; if (map[key]) { list[j] = grid[a][b]; }\n";
remend("\`\`\`ts\n" + line.repeat(1000)); // ~915ms on remend@1.3.0

That's ~915ms of main-thread time per streamed chunk when rendering a large code block.

Fix

Build the answer for every position in one linear pass (buildCodeBlockLookup, mirroring the original scan's control flow including escaped-backtick and triple-backtick handling), and cache it per text with a single-entry cache — handlers repeatedly probe the same string within one remend() call, so each probe becomes O(1) after one O(n) build. No signature changes; all six consuming handlers benefit.

Same repro after the fix: ~0.4ms (benchmark added, iterations: 10 to keep CI fast).

Behavior parity

• New __tests__/code-block-utils.test.ts includes a verbatim copy of the previous implementation as a reference oracle and asserts equality at every position across mixed inputs (fences, inline code, escaped backticks, unclosed spans), plus a cache-correctness test with alternating texts.
• Positions past text.length resolve to the end-of-scan state, matching the old loop's behavior.
• Full existing suites pass unmodified: remend 464/464, streamdown 982/982.

Notes

• Changeset included (patch).
• pnpm check / check-types clean. pnpm build succeeds for all library packages (apps/test prerender needs AI Gateway credentials I don't have as an external contributor).

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