ci: test the no-alloc build and harden the workflow

Run clippy, docs and tests for tines-core with --no-default-features,
gating the MemStore-based tests on `alloc` so they compile there. Fail on
rustdoc warnings, pass --locked everywhere, and pin the toolchain to
1.99.0. In the workflow, use read-only permissions, cancel superseded
runs, add a 20-minute timeout, and pin actions to commit SHAs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tines-core/tests/pair.rs

@@ -1,3 +1,6 @@
+// Every test here drives a `Node` over `MemStore`, which needs `alloc`.
+#![cfg(feature = "alloc")]
+
 mod common;
 use common::PairTransport;