Extract native Markdown codec from AndroidDocumentsPlugin

[Renakoni]

Summary

Sixth PR of the semantic-decomposition series, first on the native side: moves the pure-JVM Markdown byte handling out of AndroidDocumentsPlugin.java into a focused MarkdownCodec helper. Behavior-preserving — error codes, messages, and byte-level output are unchanged.

Boundary

MarkdownCodec (package-private, static, zero Android dependencies) owns:

• encoding-name normalization (the full 36-name table with the utf8 fallback),
• charset lookup with the DOCUMENT_ENCODING_UNSUPPORTED mapping,
• BOM detection (UTF-32 sniffed before its UTF-16 prefix — now documented in place) and BOM emission for the five Unicode encodings,
• strict encode/decode with CodingErrorAction.REPORT and the DOCUMENT_ENCODING_FAILED mapping,
• the 5 MB byte validation (DOCUMENT_TOO_LARGE), exposed constant included since the plugin's streaming read enforces the same limit.

The supporting value types — DocumentReadException, MarkdownWriteOptions, DecodedMarkdown — are promoted from private inner classes to package-visible top-level classes, chosen deliberately so every existing plugin reference, method signature, and catch clause compiles unchanged.

The plugin keeps everything the TODO says it must: Capacitor call handling, URI parsing, ContentResolver stream I/O (readText/writeText stay put and feed bytes through the codec), Activity lifecycle, and the mutable default-encoding/auto-detect settings — which remain plugin state passed into decode per call, so configureMarkdownSettings semantics are identical.

Testing

• New MarkdownCodecTest (8 JUnit tests, pure JVM, no emulator needed): UTF-8 round trip with CJK content, BOM write + detect, the UTF-32LE-vs-UTF-16LE sniff priority, the auto-detect-off rule that still honors a BOM matching the default encoding, the 5 MB rejection code, name normalization (unknown/null/untrimmed), and both DOCUMENT_ENCODING_FAILED paths (unmappable char on encode, malformed bytes on decode). All 8 pass via gradlew testDebugUnitTest.
• CI: the Android debug workflow now runs testDebugUnitTest before assembling, so native unit tests gate merges from this PR onward.
• compileDebugJavaWithJavac and assembleDebug green locally; the web bundle is untouched (Java-only diff), and the TS-side encoding contract is unchanged (encoding/hasEncodingBom payload fields, error-code strings).