Fix Sonar Pulse multiplayer shroud desync - #22623

Sonar Pulse can desync when its owner has explored the target water but another player has not. SpawnActorPower.Validate uses World.ShroudObscures, which depends on each client's local RenderPlayer, so clients can disagree about whether to spawn the sonar actor.

Use the current power actor owner's exploration instead. Activation, target orders and cursor selection already share this validation method. Map bounds, terrain restrictions and the AllowUnderShroud bypass retain their existing behavior.

Fixes #22621.

Validation against b6fc03fc:

• make check, make tests, make TREAT_WARNINGS_AS_ERRORS=true test and make check-scripts passed on the originally tested candidate, including all four default mods' configuration and script checks. NUnit: 508 passed, two existing PNG tests skipped. The engine source is unchanged in this update.
• Automated two-client Red Alert UI runs clicked the Sonar icon and explored water using normal mouse input and network orders. The baseline spawned sonar on one client only and both detected desync; the patched clients both spawned sonar, retained matching world hashes and continued the match. No activation/order injection or shroud override was used in this UI test.
• Separate automated checks covered owner/viewer/observer views, unexplored water, land, out-of-map targets and AllowUnderShroud behavior.

I reviewed the baseline and patched UI recordings. The game inputs were automated; this was scoped human visual review, rather than a manually played match. The UI fixture supplies a ready support-power proxy and natural camera exploration, so it does not cover the original spy-infiltration and full charge-time sequence. UI runs used isolated Linux clients; Windows/macOS gameplay was not tested.

Proposed changelog: Fixed a multiplayer desync when using Sonar Pulse on water that another player has not explored.