MP: world structures (garages/walls) TARGETABLE -- boresight ray-tests the static collision tree (task #50)

The boresight's non-mech pick only sampled the VISUAL heightfield (BTGroundRayHit),
which on arena1 is a single flat 'sky'-named ground mesh (and class-42 BuildTables
likewise holds only 'sky') -- so shots passed THROUGH the garages/walls and only mechs
moved the HUD range axis (user-reported regression: "buildings used to move the range
axis; now only mechs do, and you can only fire at a peer").

Real fix: the boresight now ALSO ray-tests the ZONE'S STATIC COLLISION SOLID TREE --
the same geometry that already blocks the mech's walk. Authentic engine mechanism:
Mover::FindBoxedSolidHitBy already tests the static world via
zone->GetCollisionRoot()->FindBoundingBoxHitBy(line). Factored its "static world" tail
into Mover::FindStaticSolidHitBy(Line*) (static solids only, no movers), wrapped by
Mech::WorldStructurePick(start,dir,range,&hit) (builds the world-space Line, reads the
entry point back via line->FindEnd since HitByBounded clips line->length=enter).

Boresight pick order is now: closest MECH (PickRayHit, damage zones + lock) -> closest
STRUCTURE (WorldStructurePick; occludes a mech BEHIND it; designates the gBTTerrainEntity
sentinel + entry point, so the range caret reads the structure distance and NO lock ring
draws, mech4.cpp:4529) -> flat ground (BTGroundRayHit) -> sky (fire-at-nothing). Also
un-skips arena1's misnamed-'sky' flat ground so the ground tier works (btvisgnd
geometry-aware skip + [mapent]/[rendent] census).

Verified headless: a BT_WSWEEP horizontal ray-fan on arena1 tracks position (3/24 hits
near the boundary -> 17-21/24 inside the interior garage cluster -> 3/24 past it =
DISCRETE interior solids, not an enclosing box), no crash/assert/AV. Interactive aim
(BTGetAimRay) can't run headless (no window -> noRay), so the sweep is the headless proof;
interactive aim is user-verified.

Note: FindBoundingBoxUnder (the ground/containedByNode BoundingBoxTree) is DOWNWARD-only
(gravity/ground-snap: *height = FindDistanceBelowBounded), useless for a horizontal
boresight; the static SOLID tree's FindBoundingBoxHitBy is the only ray-vs-world query.

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

engine/MUNGA/MOVER.cpp

@@ -1137,6 +1137,34 @@ BoxedSolid*
 	return solid;
 }
 
+//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// The "static world" tail of FindBoxedSolidHitBy on its own: ray-test ONLY the
+// zone's static solid tree (world structures -- garages, walls, props), NOT the
+// tangible movers/doors.  Same tree the mech's walk collides against, so a shot
+// designated through this lands on exactly the geometry that blocks the mech.
+// FindBoundingBoxHitBy clips line->length to the hit distance (HitByBounded ->
+// line->length = enter), so the caller reads the entry point via line->FindEnd.
+//
+BoxedSolid*
+	Mover::FindStaticSolidHitBy(
+		Line *line
+	)
+{
+	Check(this);
+	Check(line);
+
+	InterestManager *interest_mgr = application->GetInterestManager();
+	Check(interest_mgr);
+	InterestZone *zone = interest_mgr->GetInterestZone(interestZoneID);
+	Check(zone);
+	BoxedSolidTree* tree = zone->GetCollisionRoot();
+	Check(tree);
+	BoxedSolid *result = (BoxedSolid*)tree->FindBoundingBoxHitBy(line);
+	Check_Fpu();
+	return result;
+}
+
 //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 //
 BoxedSolidCollisionList*

engine/MUNGA/MOVER.h

@@ -300,6 +300,16 @@ public:
 			Line *line,
 			Entity *except_by
 		);
+	// STATIC-WORLD-ONLY ray query: the "test against the static world" tail of
+	// FindBoxedSolidHitBy factored out -- ray-tests ONLY the zone's static solid
+	// tree (the world structures: garages, walls, props), skipping the tangible
+	// movers/doors.  Used by the weapon boresight pick so a shot lands on the SAME
+	// static geometry that already blocks the mech's walk (mechs are picked
+	// separately, with their damage zones/lock).  Clips line->length to the entry.
+	BoxedSolid*
+		FindStaticSolidHitBy(
+			Line *line
+		);
 	BoxedSolidCollisionList*
 		CollideCenterOfMotion(
 			Line *line,