perf: memoize the mkdir in emitter write()

[jackyzha0]

Problem

write() in quartz/plugins/emitters/helpers.ts does a recursive mkdir for every file it writes:

const dir = path.dirname(pathToPage)
await fs.promises.mkdir(dir, { recursive: true })
await fs.promises.writeFile(pathToPage, content)

Every emitter funnels through this. A site that emits a page, an og image and an asset per note does thousands of redundant mkdir syscalls per build, nearly all of them on directories that already exist.

Change

Cache the directories we've already made. The output directory is cleaned exactly once per process — buildQuartz does rm(output, ...) at line 79 and nothing else removes directories during a build — so once a directory has been made it stays made for the life of the process. Watch mode re-imports the whole build module on a source change, which resets the map along with everything else.

The map holds the in-flight promise rather than a boolean, so two concurrent writes into the same new directory can't race a half-finished mkdir.

Verification

• tsc --noEmit clean
• npx quartz build -d docs succeeds, 313 files emitted
• output compared with and without the change: identical for all 309 deterministic files. Four files differ — index.xml, sitemap.xml, plugins/encryptedpages-demo.html, static/encryptedContentIndex.json — but a control run of two builds of unmodified code differs in exactly the same four, so that's build timestamps and the per-build encryption salt, not this change.

This is small on its own; it matters more once emitters write concurrently (see quartz-community/og-image#2).

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com

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Pull request overview

This PR reduces redundant filesystem work during builds by memoizing directory creation in the shared write() helper used by emitters, avoiding repeated mkdir(..., { recursive: true }) calls for directories that have already been created within the process.

Changes:

• Add a module-level cache (ensuredDirs) that stores in-flight/completed mkdir promises by directory path.
• Update write() to reuse the cached promise and await it before writing output files.

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