ci(codspeed): stop manufacturing spurious benchmark changes

Two CI causes of reports flagging benchmarks that the PR cannot affect.

Serialised develop baselines. All develop pushes shared one concurrency
group and never cancel, so they ran strictly one at a time. A burst of
merges leaves later commits without a finished baseline, and CodSpeed then
falls back to an older comparison base with its own footnote: "No
successful run was found on develop (X), so Y was used instead. There
might be some changes unrelated to this pull request in this report."
That footnote appears on four of twelve open-PR reports, and it lines up
with the benchmarks that flip between two fixed values in opposite
directions on unrelated PRs. Give each develop push its own group so
baselines still run on every commit, as the comment intends, without
queueing behind each other. Develop runs can now overlap, which raises
peak demand on a preemptible runner pool.

CUDA walltime suite triggered by any workflow edit. The paths filter
matched .github/workflows/**, so every dependency bump ran the walltime
CUDA benchmarks. Eleven CI-only PRs reported
cuda/bitpacked_u8/unpack/3bw[100M] improving 18-19% from an identical
base, each carrying CodSpeed's own warning that walltime on standard
hosted runners produces inconsistent data. Only this workflow defines
those jobs, so match just this file. Non-pull_request events still always
run the CUDA suite, so develop keeps full coverage.

Signed-off-by: Claude <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015ypCZPxvLuJK9FgWKF8U4P

Author: claude

.github/workflows/codspeed.yml

@@ -2,11 +2,13 @@ name: Codspeed Benchmarking
 
 # Concurrency control:
 # - PRs: new commits on a feature branch will cancel in-progress (outdated) runs.
-# - Push to develop: runs queue sequentially, never cancelled. This allows us to have benchmarks
-#   run on every commit for our benchmarks website.
+# - Push to develop: every commit gets its own group, so baseline runs never cancel and never
+#   queue behind each other. Serialising them meant a burst of merges left later commits without
+#   a finished baseline, so CodSpeed fell back to an older comparison base and reported changes
+#   unrelated to the PR being tested.
 # - `workflow_dispatch`: groups by branch and queues if run on develop.
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name == 'push' && github.sha || '' }}
   cancel-in-progress: ${{ github.ref != 'refs/heads/develop' }}
 on:
   push:
@@ -40,7 +42,8 @@ jobs:
           filters: |
             cuda:
               - "vortex-cuda/**"
-              - ".github/workflows/**"
+              # Only this workflow defines the CUDA benchmark jobs.
+              - ".github/workflows/codspeed.yml"
 
   bench-codspeed:
     strategy: