Add feedmark component: RSS/Atom/JSON Feed parser

[Copilot]

New Wasm component that parses RSS, Atom and JSON Feed input (auto-detected) into a normalized feed record, or renders it as a Markdown digest. Pure parsing, no network.

WIT

Two exports on a feed / entry record pair:

export parse: func(input: string) -> result<feed, string>;
export to-markdown: func(input: string) -> result<string, string>;

entry carries title, link, summary, content, published (RFC 3339), author — all normalized across the three source formats.

Implementation

• components/feedmark/ scaffolded after wordmark/tablemark (cdylib, wit-bindgen 0.36, workspace lints).
• Parsing delegated to feed-rs 2.3, which wraps the candidate crates suggested in the issue and handles format auto-detection.
• Markdown digest: H1 feed title, optional description, then per-entry H2 (linked when a URL is present), an italic date — by author line, and the summary (falling back to content).

Integration

• justfile: added feedmark to build, publish, versions.
• .github/workflows/publish.yml: added feedmark to the dispatch target choices, the job if: gate, and the description case.
• README.md: new row in the components table.