feat(content): retained Markdown document model, parser, and renderers [L01.01.05.10.02]

[dotnetcadet]

Summary

Turns Assimalign.Cohesion.Content.Markdown from a placeholder into the Markdown format package of the Content family — the first consumer of the Content.Text TextTokenizer (PR #931), and the prerequisite for the static content engine resource.

• Named baseline, explicit subset. The package implements a documented subset of CommonMark 0.31.2: ATX headings, paragraphs with lazy continuation, block quotes, bullet/ordered lists (tight/loose, interruption rules), fenced code, thematic breaks; inline escapes, code spans, emphasis/strong via the spec's delimiter-stack algorithm (flanking, intraword-underscore, multiple-of-three), inline links/images, URI autolinks, hard/soft breaks, numeric character references plus the five XML-predefined entities. docs/DESIGN.md carries the full subset and degradation table.
• Predictable degradation, total parsing. Excluded constructs (setext headings, indented code, raw HTML, reference links/definitions, unknown named entities, email autolinks, GFM) degrade to literal text; the parser never throws for input, and the package deliberately defines no exception root — a documented design decision, not an omission.
• Two-phase parser on the shared text layer. Block phase drives lines from TextTokenizer default options (open-container stack, tab-aware columns, container depth cap of 128); inline phase tokenizes leaf content through a custom delimiter table and assembles delimiter runs by offset contiguity.
• Model per family convention. Closed MarkdownNode family (private protected root, sealed concretes), mutable with validating setters for programmatic document construction; MarkdownText facade mirrors YamlText (Format, Parse(string|Stream|ITextContent) with encoding detection, ToHtml, Write, CreateReader/CreateWriter seams).
• Two renderers, both iterative. HTML output in the spec's shapes (tight/loose list rendering, alt-text flattening, reference-safe URL percent-encoding); canonical Markdown writer with prefix-stack container writing and conservative escaping. Both walk with explicit op stacks so adversarial inline nesting cannot overflow the call stack (verified by a 5000-deep emphasis test).
• Round-trip guarantee: for parser-produced trees, write → parse → write is a fixed point and reparse preserves rendered HTML — enforced across a 30-document theory.

Testing

142 tests, all passing: block structure (28), inlines (27), spec-derived corpus cases transcribed from CommonMark 0.31.2 with section attribution (38 theory rows), round-trip fixed-point + canonicalization (35), model/facade/seams (10+), plus degradation pins for every excluded construct. The full spec corpus harness remains #469's scope. Content.Text's 46 tests still pass; Release build is warning-free.

Wiring

• CI: Assimalign.Cohesion.Content.Markdown added to the Content workflow matrix.
• Area project map updated (libraries/Content/README.md); package README.md/docs/OVERVIEW.md/docs/DESIGN.md created.
• Solutions already carried the project; placeholder Class1.cs/UnitTest1.cs and the placeholder test csproj's forbidden <Using Include> removed.

Work items resolved by this PR

Closes #468

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