ci: mask platform-build failures in all-required (Phase 3 — mc#664)

`platform-build` has `continue-on-error: true` as a Phase 3 interim
mask while mc#664 handler test failures are in flight. In Gitea,
continue-on-error jobs report result="failure" in the needs context
(unlike GitHub Actions which reports "success"). This caused the
all-required sentinel to hard-fail on every PR.

Add PHASE3_MASKED = {"platform-build"} to the sentinel script so
platform-build failures are treated as Phase 3 suppressed. Remove
this exclusion when mc#664 is resolved and platform-build is healthy.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -568,9 +568,12 @@ jobs:
           echo "$results" | python3 -c '
           import json, sys
           ns = json.load(sys.stdin)
+          # Phase 3 masked: jobs with continue-on-error: true may report "failure"
+          # Remove when mc#664 handler test failures are resolved.
+          PHASE3_MASKED = {"platform-build"}
           # Exclude null (Phase 3 suppressed / in-flight) from the bad list.
           bad = [(k, v.get("result")) for k, v in ns.items()
-                 if v.get("result") not in ("success", None, "cancelled")]
+                 if v.get("result") not in ("success", None, "cancelled") and k not in PHASE3_MASKED]
           if bad:
               print(f"FAIL: jobs not green:", file=sys.stderr)
               for k, r in bad: