lower(strength): drop vendor-adaptive floor to OpenAI 0.10 / Google 0.15

A 2026-06-14 oracle re-test on the deployed Modal controlnet worker (v0.10.0)
cleared SynthID at OpenAI 0.10 (2 photoreal) and Google 0.15 (2 native
2816x1536, retiring the "native >= 0.30" guess), while a pixel sweep showed the
2026-06-04 cert floors (0.20/0.30) over-regenerated for no efficacy gain
(Google MAE -20% at 0.15). Lowers OPENAI_STRENGTH 0.20->0.10, GEMINI_STRENGTH
and UNKNOWN_STRENGTH 0.30->0.15.

Caveats documented in watermark_profiles.py + docs: removal near this floor is
seed-non-deterministic (a service must pin a verified seed), and the n=2 re-test
did not cover flat-graphic hard cases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_platform.py

@@ -140,11 +140,12 @@ class TestResolveStrength:
         assert resolve_strength(None, "adobe") == UNKNOWN_STRENGTH
 
     def test_ladder_is_the_certified_controlnet_floors(self):
-        # The unified ladder == the oracle-certified controlnet floors (OpenAI 0.20,
-        # Google/unknown 0.30); Google is the more-robust watermark, so it is higher.
-        assert OPENAI_STRENGTH == 0.20
-        assert GEMINI_STRENGTH == 0.30
-        assert UNKNOWN_STRENGTH == 0.30
+        # The unified ladder == the oracle-certified controlnet floors. Lowered on the
+        # 2026-06-14 Modal re-test (OpenAI 0.10, Google/unknown 0.15); Google is the
+        # more-robust watermark, so it is higher.
+        assert OPENAI_STRENGTH == 0.10
+        assert GEMINI_STRENGTH == 0.15
+        assert UNKNOWN_STRENGTH == 0.15
         assert OPENAI_STRENGTH < GEMINI_STRENGTH
 
     def test_default_strength_alias_is_unknown_vendor_value(self):