[WIP] Introduce xmdx

[jp-knj]

Summary

Introduced astro-xmdx for MDX compilation.

Changes

• Adds astro-xmdx path to @astrojs/mdx
• Keeps compatibility by preserving current behavior/fallback path where needed
• Adds benchmark methodology + results (fair vs max-performance) for review

Benchmark Setup (WIP)

Item	Value
Corpus	withastro/docs@5f58d09
Docs dir	/tmp/xmdx-bench-official/src/content/docs
Pattern	**/*.{md,mdx}
Warmup / Runs	5 / 10
Node	v22.21.1
Platform	darwin arm64
CPU	Apple M2

Results

(Lower is better)

Dataset	Files	Size (KB)	mdxjs	xmdx	speedup
small	794	507.3	241.21 ms	55.90 ms	4.31x
medium	794	2261.7	924.22 ms	271.71 ms	3.40x
large	819	12247.0	4958.02 ms	1398.20 ms	3.55x
full-all	2407	15015.9	6405.87 ms	1722.59 ms	3.72x

Profiling Notes

• JS CPU profiling indicates @mdx-js/mdx time is concentrated in micromark / mdast-util-from-markdown
  stack.
• xmdx time is mostly inside native compile entry (Rust-side internals not fully visible in V8 cpuprofile).

CI Benchmark (withastro/docs full build)

From jp-knj/xmdx#25:

Command	Mean [s]	Min [s]	Max [s]	Relative
@astrojs/mdx	383.694 ± 4.463	378.624	387.032	1.17 ± 0.04
astro-xmdx	328.865 ± 10.096	321.346	340.339	1.00

Notes for reviewers

• Numbers are corpus- and environment-specific.
• Median-based comparison is used for stability.
• This PR’s goal is to validate astro-xmdx integration viability and performance impact in @astrojs/mdx.

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es/recipes/making-toolbar-apps.mdx	Localization changed, will be marked as complete.
ja/guides/testing.mdx	Localization changed, will be marked as complete.
ru/guides/testing.mdx	Localization changed, will be marked as complete.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.