perf: replace MDAST tag extraction with a linear PascalCase scan

[erunion]

Summary

• Replace full MDAST parse + runSync in mdxishTags and RMDX tags with a shared linear PascalCase scanner (scanPascalCaseTags)
• Skip fenced/inline code (and magic blocks for MDXish); RMDX excludes builtins coerced by readmeComponentsTransformer
• Fixes severe time/memory cost on large docs

Benchmarks

Measured with Vitest on Node, cold call + 5 steady-state calls (median). Memory deltas are process.memoryUsage() before/after the cold call (heap used / RSS).

Fixture A — real Artifactory release notes (artifactory-self-managed-releases.md, ~719KB)

Primary case that motivated this change (~95 <Callout>, ~180 <Anchor>, GFM tables).

API	Metric	Before	After	Change
mdxishTags	cold latency	799 ms	1.5 ms	~547× faster
mdxishTags	steady median	702 ms	1.2 ms	~585× faster
mdxishTags	heap used Δ	+61 MB	+3.0 MB	~20× less
mdxishTags	RSS Δ	+58 MB	+2.8 MB	~21× less
tags (RMDX)	—	throws (invalid JSX / micromark-extension-mdx-jsx)	1.1 ms, +3 MB heap Δ	after succeeds; before cannot parse this MDXish doc

Fixture B — synthetic valid MDX (~932KB, 2800× Callout/Anchor/CustomWidget/TableBlock)

Used so RMDX tags can be compared apples-to-apples (strict MDX).

API	Metric	Before	After	Change
mdxishTags	cold latency	2500 ms	2.3 ms	~1068× faster
mdxishTags	steady median	2568 ms	1.8 ms	~1411× faster
mdxishTags	heap used Δ	+125 MB	+1.8 MB	~69× less
mdxishTags	RSS Δ	+122 MB	~0 MB	~122 MB saved
tags (RMDX)	cold latency	12 486 ms	2.9 ms	~4233× faster
tags (RMDX)	steady median	13 931 ms	1.7 ms	~8204× faster
tags (RMDX)	heap used Δ	+205 MB	~0 (GC noise)	orders of magnitude less
tags (RMDX)	heap used after	307 MB	56 MB	~5.5× lower
tags (RMDX)	RSS after	638 MB	180 MB	~3.5× lower

Test plan

• __tests__/lib/mdxishTags.test.ts and __tests__/lib/tags.test.ts green (including <<VARIABLE>>, nested tags, Table semantics)
• Before/after bench on Artifactory release notes + synthetic large MDX (numbers above)
• Confirm consumers that call tags / mdxishTags still get expected component lists for custom components