fix(mdx): stop double-escaping angle brackets inside code

[nlimpid]

Problem

The preflight rewrites in astro.config.ts run over the raw mdx source with code blocks included. Rule 5 turns <iostream> and <token> into <…>, the downstream pipeline escapes that a second time, and the page renders the entity text:

#include <iostream>          ← every C++ SDK example
Bearer <token>              ← every curl example

419 pages are affected. Anyone copying a snippet off the page gets broken code.

It is not only rule 5 — rules 6, 7, 9 and 10 rewrite code-region content too, so fixing a single rule is not enough.

Fix

Stash the code regions before rules 2-10 run, restore them afterwards. Three kinds, and the order matters:

1. the CliCommand payload built in rule 1.5 (a JSON string that may itself contain backticks, so it goes first)
2. fenced blocks (before inline spans, or a fence's own backticks get read as a span)
3. inline code spans

The placeholder uses U+E000 rather than NUL so oxlint's no-control-regex stays quiet.

Verification

Against the dev server:

check	result
C++ fence	#include <iostream> ✅
curl fence	Bearer <token> ✅
inline span	<client_id> ✅
all 78 absolute-slug English pages	200, no render failure
rule 9 REST placeholder	/v1/content/{symbol}/news unchanged
rule 1.5 CliCommand	terminal card unchanged
rule 10 directive	callout unchanged
oxlint	clean

The regression checks matter most here: had the stash regex over-captured into prose, those placeholders would reach MDX as undefined components and the pages would 500. All 78 returning 200 confirms the prose-side escaping still works.