Make embedded skeleton Clippy-clean

[noise64]

• resolves https://linear.app/golem-cloud/issue/GOL-421/make-the-embedded-skeleton-clippy-clean-and-enforce-it-in-ci
• adds one failure-safe local and CI Clippy helper covering P2/P3 default and maximal feature lanes with -Dwarnings
• restores Cargo.toml_ before best-effort artifact cleanup and adds success, early-failure, and late-failure contract coverage
• fixes the existing skeleton warnings without broad allowances while preserving the SQLite JavaScript bridge ABI and module-loader behavior
• fixes P3 WebSocket receive/close reentrancy by releasing RefCell ownership before awaiting the host receive
• documents the shared skeleton lint workflow for contributors and agents

[noise64]

Correction SHA 311ef670e2e2df597c7272d9a54c87bfcd6f19b9 is fully green in CI run 33259322804.

The dedicated skeleton-clippy job passed all failure-safe embedded-skeleton lanes, and the build, test-other, P2/P3 runtime, P2/P3 node-compat, and dedicated P3 suites all passed. The initial test-runtime-p3 (2) transport flake passed on rerun without a source change.

Local verification also passed the four-lane helper and confirmed that Cargo.toml_ was restored and generated skeleton artifacts were removed after every lane.