docs: rewrite README to reflect wired read/write and real conformance numbers

readMarkdown/writeMarkdown/markdownCodec were already implemented, but the
README still described them as placeholders and claimed the conformance
exclusion list was empty when it in fact names 192 examples.

Replace the Status/Usage/Architecture sections with the current wiring,
add a Gotchas section enumerating every MarkdownDiagnosticCodes
construct-mapping gap, and add a Fidelity section with the real measured
CommonMark/GFM round-trip pass rates (70.7%/95.7%/71.6% combined) and the
inverse framing from pdf-codec's own Fidelity section: here the target
schema, not the source parser, bounds fidelity.

Author: Mearman