Fix broken heading links

[rohitpaulk]

Problem

On https://ghostty.org/docs/features/ssh, the in-page [shell integration](#shell-integration) link is broken. It points to #shell-integration, but the heading on that page actually renders with id="shell-integration-2".

Root cause

In src/lib/docs/remark-heading-ids.mjs, the encounteredIDs Map used to de-duplicate heading IDs is created in the plugin factory scope:

export default function remarkHeadingIds() {
  const encounteredIDs = new Map(); // shared across ALL docs
  return (node) => { ... };
}

Next.js instantiates the remark plugin once (see next.config.mjs) and reuses it for every MDX file, so the counter persists across documents. Multiple docs contain a "Shell Integration" heading (docs/features/ssh.mdx, docs/install/release-notes/1-3-0.mdx, docs/install/release-notes/1-3-1.mdx), so they get slugs like shell-integration-2, shell-integration-3 etc.

Fix

Move the encounteredIDs Map inside the returned per-document transformer so the counter resets for each file.

Validation

Verified with a real npm run build and inspection of the generated static HTML:

• docs/features/ssh.html: heading id goes from shell-integration-2 to shell-integration (i.e. previously broken link now works)
• docs/install/build.html (two ### Dependencies headings): still correctly produces dependencies and dependencies-2

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