test(perf): 2.5D backdrop twin profile judges one update pass per floor switch (#743)

No Full Performance profile walked the backdrop (imagePlan) path: every
large-house fixture has plan_url null. So the #739 K1 double render -- a
warm 2.5D floor switch with a backdrop cleared the ready paper, inserted
the veil, probed the card background and rendered a second time -- was
invisible to CI by time and structurally; a temporary probe found it.

large-house-isometric-backdrop-v1 is the twin of large-house-isometric-v1
with the shipped f1.svg under a URL of its own on every floor
(plan_aspect 1, room geometry unchanged). The variant is derived in the
runner as plan-snap's is, so demo/fixtures and the bundle fingerprint do
not change. A first stable frame without the backdrop image fails the
sample. After the switchCycle window and its #735 guard, before forced
GC and outside every timed window, a probe makes six warm switches and
counts performUpdate passes until updateComplete resolves true: the K1
second pass starts after the first updateComplete resolves, so a count
taken right after the first await reads one on both sides.

evaluate.mjs rejects a candidate of this profile unless every switch took
one pass, or when the probe is missing; the base is reported, not judged
(v1.78.0 and dev before #739 take two). The budget is a copy of the
historical isometric budget under the new profile id. performance.yml
gains the isometric-backdrop matrix entry with exact-SHA comparison.

Witness: a tree with #739 reverted reads perSwitch 2 in every sample and
benchmark:compare against the branch report throws on the pass count;
v1.78.0 reads 2, the branch reads 1.

Issue: #743
User-Visible: no
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd

.github/workflows/performance.yml

@@ -40,6 +40,7 @@ jobs:
         profile:
           - large-house
           - isometric
+          - isometric-backdrop
           - isometric-stage3
           - plan-snap
           - interaction
@@ -134,6 +135,10 @@ jobs:
               npm run benchmark:large-house-isometric -- --target-root=../baseline --samples=7 --warmups=1 --output=../artifacts/performance/isometric-baseline.json
               npm run benchmark:large-house-isometric -- --target-root=. --samples=7 --warmups=1 --output=../artifacts/performance/isometric-candidate.json
               ;;
+            isometric-backdrop)
+              npm run benchmark:large-house-isometric-backdrop -- --target-root=../baseline --samples=7 --warmups=1 --output=../artifacts/performance/isometric-backdrop-baseline.json
+              npm run benchmark:large-house-isometric-backdrop -- --target-root=. --samples=7 --warmups=1 --output=../artifacts/performance/isometric-backdrop-candidate.json
+              ;;
             isometric-stage3)
               npm run benchmark:isometric-stage3-dense -- --allow-stage2-base --target-root=../baseline --samples=7 --warmups=1 --output=../artifacts/performance/isometric-stage3-baseline.json
               npm run benchmark:isometric-stage3-dense -- --target-root=. --samples=7 --warmups=1 --output=../artifacts/performance/isometric-stage3-candidate.json
@@ -188,6 +193,9 @@ jobs:
             isometric)
               npm run benchmark:compare -- --budgets=demo/performance/budgets-large-house-isometric.json --baseline=../artifacts/performance/isometric-baseline.json --candidate=../artifacts/performance/isometric-candidate.json --baseline-sha="$(git -C ../baseline rev-parse HEAD)" --candidate-sha="$(git rev-parse HEAD)" --output=../artifacts/performance/isometric-comparison.json
               ;;
+            isometric-backdrop)
+              npm run benchmark:compare -- --budgets=demo/performance/budgets-large-house-isometric-backdrop.json --baseline=../artifacts/performance/isometric-backdrop-baseline.json --candidate=../artifacts/performance/isometric-backdrop-candidate.json --baseline-sha="$(git -C ../baseline rev-parse HEAD)" --candidate-sha="$(git rev-parse HEAD)" --output=../artifacts/performance/isometric-backdrop-comparison.json
+              ;;
             isometric-stage3)
               npm run benchmark:compare -- --budgets=demo/performance/budgets-isometric-stage3-dense.json --baseline=../artifacts/performance/isometric-stage3-baseline.json --candidate=../artifacts/performance/isometric-stage3-candidate.json --baseline-sha="$(git -C ../baseline rev-parse HEAD)" --candidate-sha="$(git rev-parse HEAD)" --output=../artifacts/performance/isometric-stage3-comparison.json
               ;;