Add internal Markdown parsing

docs/internal/validation.md

@@ -45,6 +45,15 @@ matching, then returns the first oversized path with `FileSizeMeasurement`.
 lazily, and resolves destinations without consulting the worktree or
 neighboring files.
 
+The internal Markdown parser uses Goldmark to record inline, reference, and
+autolinks, images, titles, and their source use sites, and to build heading
+anchors from visible AST text. It supports ATX and Setext headings, decodes
+Markdown text through the parser, and assigns collision-free numeric suffixes
+to repeated anchors. Link locations use one-based line and byte-column values.
+A per-path cache retains the parsed records and anchors for each candidate
+document. The parser remains internal; destination resolution and check selection
+are tracked in the [implementation plan](../roadmap/implementation.md).
+
 Committed-tree metadata and blob reads disable lazy fetching and Git transports
 for those child commands. Missing local objects produce inspection errors;
 source preparation retains its separate network-capable operations. See Git's
@@ -117,6 +126,6 @@ symlink expansion detects cycles, and traversal honors cancellation. Absolute
 destinations and symlink targets within the repository directory map back to
 candidate-relative paths; existence still comes from committed entries.
 
-The [Markdown roadmap](../roadmap/v0.2.0.md#8-offer-offline-markdown-link-validation)
-owns proposed link syntax, anchors, diagnostics, and boundary policy. This
-foundation provides committed-file and destination-resolution primitives.
+The [implementation plan](../roadmap/implementation.md) owns Markdown
+destination checking, diagnostics, and boundary policy. The committed-file and
+destination-resolution primitives are available for that later integration.