Add Markdown HTML publication

docs/internal/transform.md

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+# Transform
+
+## Purpose
+
+`internal/transform` defines generated publication artifacts. `internal/transform/markdown` implements Markdown-to-HTML sidecar generation.
+
+## Inputs and outputs
+
+Inputs are a validated source bundle and source backend. Outputs include destination path, source path, transform id, generated bytes, SHA-256, and size.
+
+## Markdown behavior
+
+Markdown files ending in `.md` generate `.html` files in the same logical directory. Non-Markdown files do not generate outputs. Raw HTML embedded in Markdown is not passed through by the renderer.
+
+Generated HTML is deterministic for the same source content and transform configuration.
+
+## Boundaries
+
+Transforms do not publish files, mutate source bundles, or write destination state. Publish planning selects and writes transform outputs.
+
+## Tests
+
+Before changing transform behavior, inspect tests under `internal/transform`.