Prepare facet-xml 0.50.0-rc.0

[fasterthanlime]

Bumps the facet-xml workspace to 0.50.0-rc.0 and aligns Facet/rediff dependencies to the published 0.50.0-rc.0 stack.

This also carries the CI migration already prepared locally: Bearcove Linux runners, actionlint runner labels, Node 24-safe checkout/rust-cache actions, and composite-only nested actions. Facet 0.50 made facet-dessert enums non-exhaustive, so the deserializer now has explicit fallback behavior for future PointerAction/DessertError variants. README dependency snippets were updated to the RC version.

Verification:

• actionlint
• capn
• cargo clippy --workspace --all-features --all-targets -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps
• cargo update --workspace --locked
• cargo +1.90 check --workspace
• cargo nextest list
• cargo nextest run (372 tests)
• push hook: cargo-shear, clippy, docs, build tests, run tests

Action runtime audit:

• actions/checkout@v6 uses node24
• Swatinem/rust-cache@v2 uses node24
• dtolnay/rust-toolchain, taiki-e/install-action, release-plz/action and nested release-plz actions are composite/shell actions