Improve Rust JPEG 2000 discovery

[jcwal1516]

What changed

• gives the repository and public crate explicit Pure-Rust JPEG 2000 and HTJ2K naming
• points crate metadata and packaged documentation to the focused codec landing page
• makes /rust-jpeg2000-codec/ the primary generic-query target while keeping the homepage project-focused
• prioritizes the target in the sitemap and adds a regression policy test

Why

The existing metadata split generic Rust JPEG 2000 relevance across the repository homepage and several documentation pages. This change consolidates those signals around one canonical landing page.

Validation

• cargo fmt --all -- --check
• cargo test -p xtask --test repo_lint public_docs_policy
• cargo check -p j2k
• cargo test -p j2k --doc
• cargo package -p j2k --allow-dirty --no-verify --list
• xmllint --noout docs/sitemap.xml
• git diff --check