Add Markdown parser support (.md, .markdown)

[josephismikhail]

Problem

Ix does not currently parse .md or .markdown files. Markdown files are treated as
unrecognized and excluded from analysis, meaning documentation, API references, guides,
and developer-facing content are invisible to the system.

Proposed solution

Add a Markdown parser that can identify and extract:

• Heading entities at all levels (h1–h6), including ATX (#) and setext (===, ---) syntax
• Heading hierarchy as CONTAINS relationships (file → h1 → h2 → h3)
• Section chunks scoped to each heading
• YAML frontmatter blocks
• VitePress/docusaurus anchor syntax (e.g. ## Heading {#anchor-id}) cleaned from entity names

Support the following file patterns: *.md, *.markdown

Why it matters

Markdown is the standard format for documentation in software projects — READMEs, API
references, changelogs, guides, and architecture docs are almost universally written in
it. Without parsing Markdown, Ix cannot reason about a project's documentation layer,
answer questions grounded in written specs, or surface relationships between docs and
the code they describe. This is especially relevant for documentation-heavy repos like
Vue, React, and other OSS projects where .md files are a primary artifact.

Merge requirements

Before this can be merged to main, the following must pass:

• Tested on 3 repos
• No regressions on existing parsers (ix map on a known TS/JS repo, verify counts unchanged)
• Unit + smoke tests pass (npm test in ix-cli)
• Heading hierarchy chains correctly: file → h1 → h2 → h3
• Both ATX and setext heading syntax ingested
• ix contains returns children for a known markdown file and heading
• Frontmatter entities present where expected
• Section chunks created alongside heading entities
• ix text returns results with language: markdown
• Both .md and .markdown extensions ingested