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 In the workflow, use read-only permissions, cancel superseded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};

tines-core/src/store.rs

@@ -2,9 +2,9 @@
 //!
 //! This module defines the [`Store`] trait plus its [`TrimPolicy`] and
 //! [`StoreError`] types. Everything here is `#![no_std]`-clean and does **not**
-//! require the `alloc` feature — only concrete backends (see
-//! [`mem_store`](crate::mem_store)) need allocation, so the trait, policy, and
-//! error types remain available on `no_std` targets as well.
+//! require the `alloc` feature — only concrete backends (such as `mem_store`,
+//! which is compiled only with `alloc`) need allocation, so the trait, policy,
+//! and error types remain available on `no_std` targets as well.
 
 use crate::observation::{Observation, ObservationId, Tick};