feat: support custom starting line number in code blocks (#287)

[sleitor]

Summary

Implements the feature requested in #287: support for custom starting line numbers in fenced code blocks via a startLine=N meta option.

Thanks to @haydenbleasel for the CSS workaround hint in the issue — this PR formalises it as a first-class feature!

Usage

```js startLine=10
const x = 1;
const y = 2;
```

Line numbers will begin at 10 instead of the default 1.

How it works

1. remarkCodeMeta remark plugin (lib/remark/code-meta.ts) — forwards the fenced-code meta string to the hast code element as a metastring property, so it is available to the custom React code component.
2. Sanitize schema — the default rehype-sanitize schema is updated to allow the metastring attribute on code elements.
3. CodeComponent (lib/components.tsx) — parses startLine=N from metastring and passes it as a startLine prop to CodeBlock.
4. CodeBlockBody (lib/code-block/body.tsx) — applies counter-reset: line N-1 as an inline style when startLine > 1. This overrides the Tailwind [counter-reset:line] class and makes CSS counters begin from the specified number.
5. CodeBlock / HighlightedCodeBlockBody — the startLine prop is threaded through the component tree.

Changes

• packages/streamdown/lib/remark/code-meta.ts — new remark plugin
• packages/streamdown/index.tsx — import plugin, update sanitize schema, add to defaultRemarkPlugins
• packages/streamdown/lib/components.tsx — parse startLine from meta, pass to CodeBlock
• packages/streamdown/lib/code-block/index.tsx — add startLine prop
• packages/streamdown/lib/code-block/highlighted-body.tsx — pass startLine through
• packages/streamdown/lib/code-block/body.tsx — apply counter-reset style
• packages/streamdown/__tests__/code-block-start-line.test.tsx — 12 new tests
• .changeset/code-block-start-line.md — changeset

Tests

12 new tests covering:

• remarkCodeMeta unit tests (plugin attaches/omits metastring correctly)
• CodeBlockBody with startLine prop (correct CSS counter-reset value)
• CodeBlock integration (renders with correct CSS for various startLine values)

All 804 tests pass.

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