Weapons: Fix muzzle below terrain reporting free line of fire

[eun-ice]

Units whose muzzle is clipping inside terrain stop moving and try to shoot, because CWeapon::HaveFreeLineOfFire and CCannon::HaveFreeLineOfFire return the shot as unobstructed if it starts below terrain.

Partly addresses #3242.
Likely addresses #3301 (see below).

Problem

CGround::LineGroundCol returns a hit distance of 0 when the ray origin is below the terrain. TraceRay only accepted ground hits with groundLength > 0.0f, and CWeapon::HaveFreeLineOfFire applied the same > 0 filter to the returned distance. A ray that starts underground was therefore reported as unobstructed. CCannon::HaveFreeLineOfFire had the identical pattern with CGround::TrajectoryGroundCol, which also reports 0 for that case.

Consequences:

• A Sheldon standing against a cliff with its muzzle 65 elmo inside the rock passes the pre-aim line-of-fire test, stops, and can never fire because the fire-time test rejects the underground muzzle (Attack Command: Attackers can stop without being able to fire when obstructed by terrain #3242, the "stops and does not even start to walk" case).
• Spring.GetUnitWeaponHaveFreeLineOfFire tells game code the same thing.

Additionally the underground test in LineGroundCol compared the origin against the corner vertex of its heightmap square rather than the interpolated surface. Next to a steep cliff that vertex can sit far above an origin that is well clear of the ground, and then the whole ground trace was skipped. That is the shape of #3301: a mech at the foot of a cliff reports a free line of fire to a unit on top of it as soon as it moves close to the cliff face. Spring.TraceRayGroundBetweenPositions, which @FLOZi swapped in, treats 0 as a hit and so looked correct in the same situation.

Change

• TraceRay: accept a ground distance of 0 as a hit (the ray is blocked at its origin). Only -1 means "no ground hit".
• CWeapon::HaveFreeLineOfFire: same for the returned distance. CMissileLauncher already handled it.
• CCannon::HaveFreeLineOfFire: reject a source below GetHeightReal explicitly, the test the fire-time check already applies to the muzzle, and keep ignoring the 0 from TrajectoryGroundCol. That scan uses GetApproximateHeight, which on rough ground lies above sources a couple of elmo clear of the interpolated surface; accepting its 0 blocked 48 above-ground probe points that are free today.
• CGround::LineGroundCol: the underground test compares against the interpolated height (InterpolateCornerHeight) and treats an origin exactly on the surface as above ground. LineGroundSquareCol still returns a hit at distance 0 when such a ray points into the ground, so the "unit position on the surface" case keeps working for rays pointing up and is blocked for rays pointing down.

Callers that scan ground with TraceRay are HaveFreeLineOfFire, BeamLaser, LightningCannon, Rifle and the AI callbacks. The weapons already refuse to fire when the real muzzle is below GetHeightReal (TryTarget, preFire), so with the interpolated underground test they never reach the new "blocked at origin" branch at fire time.

#3241 is stacked on this PR. Its explicit below-ground check for the predicted muzzle stays: it also covers weapons that skip ground checks (avoidGround = false) and mirrors the fire-time test.

Validation

Headless probe in the #3242 rock scenario (five Sheldons behind the rock on All That Glitters, one with its muzzle inside the cliff; gadget dbg_lof_underground_probe.lua in my workspace), same BAR checkout, before = engine without this commit:

The probe creates a Flea (BeamLaser, CWeapon::HaveFreeLineOfFire) and a Pawn (Cannon, CCannon::HaveFreeLineOfFire) and calls Spring.GetUnitWeaponHaveFreeLineOfFire with explicit source positions towards the Fatboy.

test	before	after
Sheldon with its muzzle 65 elmo inside the cliff: 3-arg and 6-arg (from muzzle) call	free / free	blocked / blocked
the other four Sheldons (muzzle 11 below to 26 above ground)	blocked	blocked (unchanged)
source 40 elmo below each Sheldon's position, laser and cannon (10 calls)	all free	all blocked
source 40 elmo above each Sheldon's position, laser and cannon	all blocked (rock in the way)	all blocked (unchanged)
Spring.TraceRayGroundBetweenPositions from the same underground sources	hit at 0	hit at 0 (unchanged)
laser, 289 sources 2 elmo above the ground on a 128x128 grid around the cliff Sheldon	8 free: exactly the 8 points whose heightmap corner vertex lies above the source, i.e. where the old underground test skipped the trace	0 free: those 8 rays hit the rock face within 2.3 elmo
cannon, same grid	48 free	48 free, identical set (the trajectory scan is unchanged)

Full rock reproducer (hold position and maneuver) on master + this commit: the cliff Sheldon's pre-aim test now fails (tryTarget=false lofAim=false lofMuzzle=false), so it no longer stops believing it has a shot. Getting it to walk out of the corner is what #3241 does; without it master's CommandAI still leaves it there.