Move CI planning to cargo-rail-action v10 and cargo-rail 0.29

[Tryanks]

Supersedes #463.

loadingalias/cargo-rail-action v10 is a rewrite: it installs Cargo-Rail from the ≥0.26 component archives (so the pinned 0.25.0 404s on SHA256SUMS, which is the failure on #463), requires plan contract v9 (Cargo-Rail 0.28+), replaces the Python plan.py reader with a cargo-rail-action plan … launcher, and no longer exposes a plan-reader output (version, plan-file and required-work remain; since/all inputs are unchanged).

What changed:

• Plan job: action pinned to v10.0.1 (4a8df18) with version: 0.29.0. The exact --from/--to re-plan stays, but required_work now comes from cargo-rail-action plan required. The continue-on-error + plan.py fallback are gone: Cargo-Rail 0.29.0 plans a --since diff that contains renames (verified locally on a scratch commit renaming a doc and a Rust module), so the comment about the 0.25.0 "not strictly sorted" bug no longer applies.
• Desktop jobs: Cargo-Rail 0.29 binds a saved plan to the planning host's Cargo executable/toolchain identity (verify_saved_plan in commands/plan.rs), and every v10 selector re-verifies before emitting. A Linux plan can therefore no longer be consumed on macOS/Windows, which the action's README also calls out. desktop-check now runs the same planner action after the toolchain is installed (skipped when host-full widens the scope to --workspace) and reads scope from its own plan; the ci-plan artifact, install_cargo_rail.py (0.25.0 checksums) and the plan-verify step are removed. The plan job exports base so consumers plan the same base..head pair.
• run_rail_cargo.py calls cargo-rail-action plan cargo-scope|cargo-args instead of the --reader script; test_ci_scope.py fakes that launcher and expects 0.29.0.

Local evidence:

• CARGO_RAIL_BIN=<cargo-rail 0.29.0> python3 .github/scripts/test_ci_scope.py → 13 tests OK.
• With cargo-rail 0.29.0 and the v10.0.1 cargo-rail-action runtime downloaded from the releases: cargo rail plan --json --from origin/main --to HEAD, cargo-rail-action plan required|cargo-scope|cargo-args, and cargo rail plan --verify all succeed on this repo; a --since <sha> plan also succeeds in a --depth 1 clone with only the base commit fetched (the shape the desktop jobs will see) and selects the same packages as the exact plan; a worktree plan still verifies after a cargo check populates target/.
• CI on this PR is the authoritative check for the per-platform planning.