CI: raise nextest slow-timeout to 300s for coverage's instrumented recompile

The Code Coverage job runs under llvm-cov; its subprocess example tests
(tests/suites/examples.rs) inherit `-C instrument-coverage` and recompile the
examples instrumented, which legitimately takes >120s. A non-instrumented
pre-build cannot match that fingerprint, so the fix is headroom, not a
pre-build step: bump terminate-after 3 -> 5 (180s -> 300s). The Test job is
unaffected (its pre-build keeps those tests sub-second); true hangs are still
bounded, now at 5 minutes instead of 3.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: isPANN

.config/nextest.toml

@@ -2,15 +2,20 @@
 #
 # Project policy is that no single test exceeds 5s (see .claude/CLAUDE.md).
 # slow-timeout is a CI safety net far above that budget: a test is flagged slow
-# every 60s and forcibly terminated after 3 periods (180s). This bounds a truly
+# every 60s and forcibly terminated after 5 periods (300s). This bounds a truly
 # hung test (cf. issue #1069, where big_o expansion looped indefinitely and the
 # whole job was SIGTERM-killed at the runner level) — nextest reports it as a
 # clean per-test timeout failure instead of stalling the job.
 #
-# The headroom (180s, not a tight ~10s) is deliberate: the subprocess example
-# tests in tests/suites/examples.rs may recompile under a different feature set.
-# CI pre-builds those examples (see ci.yml) so they run in a few seconds, but
-# the generous bound keeps the gate robust if that pre-build is ever skipped
-# (e.g. local `cargo nextest run`).
+# The large headroom (300s, not a tight ~10s) is deliberate. The subprocess
+# example tests in tests/suites/examples.rs shell out to `cargo run --example
+# … --features ilp-highs`:
+#   - In the Test job, CI pre-builds those examples (see ci.yml) so the
+#     subprocess reuses artifacts and each test runs in well under a second.
+#   - In the Code Coverage job, the subprocess inherits llvm-cov's
+#     `-C instrument-coverage` RUSTFLAGS, so it recompiles the examples
+#     *instrumented* (a non-instrumented pre-build would not match its
+#     fingerprint, so pre-building there is pointless). That instrumented
+#     recompile legitimately takes >120s, hence the 300s bound.
 [profile.default]
-slow-timeout = { period = "60s", terminate-after = 3 }
+slow-timeout = { period = "60s", terminate-after = 5 }