ci: PR shard profile (x86 and Windows whole, arm64 x2)

_test.yml takes a shard_profile input: "standard" (the default; dry run and
release pass nothing and keep ubuntu x3 / windows x2, byte-identical
matrices) or "pr", which pr.yml now uses: ubuntu x86 and Windows run as one
job each and only arm64, the slowest leg, splits in two. A PR runs few legs
and every shard re-pays its setup, so fewer, longer jobs cost less. An
unknown profile is an error. The wiring contract pins both the default and
PR's use of the profile.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

.github/workflows/_test.yml

@@ -16,6 +16,10 @@ on:
         description: 'Split each C-suite leg across parallel shard jobs (ubuntu x3, windows x2); off = one job per leg, byte-identical to the pre-shard topology'
         type: boolean
         default: false
+      shard_profile:
+        description: 'Shard counts when shard_suites is on: standard = ubuntu x3, windows x2 (dry run, release); pr = ubuntu x86 x1, ubuntu arm64 x2, windows x1 (fewer jobs re-paying setup)'
+        type: string
+        default: standard
       lanes:
         description: 'PR lane selection (scripts/ci/select-lanes.sh JSON list): only those legs and jobs run; "all" = every one (dry run, release)'
         type: string
@@ -44,6 +48,7 @@ jobs:
         env:
           BROAD: ${{ inputs.broad_platforms }}
           SHARDS: ${{ inputs.shard_suites }}
+          SHARD_PROFILE: ${{ inputs.shard_profile }}
           LANES: ${{ inputs.lanes }}
         run: |
           # Every CORE leg is tagged with the lane (scripts/ci/select-lanes.sh)
@@ -113,14 +118,24 @@ jobs:
           # shards); Windows gets 2 (every extra shard re-pays ~5 min of
           # MSYS2 setup); macOS stays at 1 — mac legs are not the critical
           # path and mac runner concurrency ceilings are the tightest.
+          # The pr profile shards less: a PR runs few legs and every shard
+          # re-pays its setup, so x86 and Windows run whole and only arm64,
+          # the slowest leg (~72 min unsharded), splits in two.
+          case "$SHARD_PROFILE" in
+            standard) N_X86=3; N_ARM=3; N_WIN=2 ;;
+            pr) N_X86=1; N_ARM=2; N_WIN=1 ;;
+            *) echo "::error::unknown shard_profile '$SHARD_PROFILE'"; exit 1 ;;
+          esac
           if [ "$SHARDS" = "true" ]; then
-            UNIX=$(jq -c '[.[] | . as $e
-              | (if ($e.os | startswith("ubuntu")) then 3 else 1 end) as $n
+            UNIX=$(jq -c --argjson x86 "$N_X86" --argjson arm "$N_ARM" '[.[] | . as $e
+              | (if ($e.os | startswith("ubuntu")) then
+                   (if ($e.os | endswith("-arm")) then $arm else $x86 end)
+                 else 1 end) as $n
               | range(1; $n + 1) as $i
               | $e + {shard: "\($i)/\($n)"}]' <<< "$UNIX")
-            WIN=$(jq -c '[.[] | . as $e
-              | range(1; 3) as $i
-              | $e + {shard: "\($i)/2"}]' <<< "$WIN")
+            WIN=$(jq -c --argjson n "$N_WIN" '[.[] | . as $e
+              | range(1; $n + 1) as $i
+              | $e + {shard: "\($i)/\($n)"}]' <<< "$WIN")
           fi
           echo "unix={\"include\":$UNIX}" >> "$GITHUB_OUTPUT"
           echo "windows={\"include\":$WIN}" >> "$GITHUB_OUTPUT"

.github/workflows/pr.yml

@@ -53,10 +53,12 @@ jobs:
       # Perf assertions are timing-sensitive on shared runners; they stay in
       # dry runs and releases where a flaky red does not block a merge.
       skip_perf: true
-      # Iteration speed: split each C-suite leg across parallel shard jobs
-      # (ubuntu x3, windows x2; coverage re-proven every run by the
-      # shard-completeness job). Dry runs and releases shard identically.
+      # Iteration speed: split C-suite legs across parallel shard jobs
+      # (coverage re-proven every run by the shard-completeness job). The PR
+      # profile shards only arm64 (x2); x86 and Windows run whole, since each
+      # shard re-pays its setup. Dry runs and releases keep x3 / x2.
       shard_suites: true
+      shard_profile: pr
 
   # ── Which lanes does this PR need? The changed files go through
   #    scripts/ci/select-lanes.sh (tier, lanes, full, reasons); docs / CI /