fix(ci): pin repaired Stack integration

[XnLemon]

Summary

• pin Satellite Integration to NeKiro-Stack 12a651e887079b0b3ad40fa7c2fd21371d2ee935
• pass the same immutable Stack SHA into the reusable workflow
• restore backend/browser assembly after the secure Nacos fixture was added

The Stack fix is merged in NeKiro-Stack #13 and its reusable workflow passed backend/browser acceptance against Core 8c73ac7.

Validation

• go build ./...
• go test -count=1 ./...
• go vet ./...
• git diff --check
• Stack regular CI: 31418840644
• Stack reusable Core integration: 31418980666

Fallback delta: removed 0, retained 0, added 0, net 0
Added fallback evidence: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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