9e7af1080e15c9ada108a705e72d8c1377385904  Add initial `@environment-map` support for Spatial Portals

Classified as not security-relevant.
Triage note: Pure feature work behind ENABLE(SPATIAL_PORTAL): adds CSSEnvironmentMapRule.idl, new EnvironmentMapLoader.cpp, StyleRuleEnvironmentMap, and parser plumbing (consumeEnvironmentMapRule/Descriptor). No bug addressed.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), security_assert (+3.0), null_guard (+2.59), uaf_lifetime (+2.2), msg_noise (-2.5), msg_feature (-1.0), area:Source/WebCore/css (+2.0), large_churn (-2.0)