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 In the workflow, use read-only permissions, cancel superseded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/src/exchange.rs

@@ -572,7 +572,7 @@ fn insert_pos(heap: &Vec<(ObservationId, u64), { consts::SEND_QUEUE_CAP }>, age:
     pos
 }
 
-#[cfg(test)]
+#[cfg(all(test, feature = "alloc"))]
 mod tests {
     use super::*;
     use crate::{MemStore, Observation, Payload, Topic};