Camera-relative wall climb up/down keys to fix grab/release twitch

When running backwards toward the camera into a wall, the grab decided on
the camera-relative world direction (S pushes into the wall -> grab) while
the base release decided on the raw S axis (v < 0 -> immediately let go),
causing a per-frame grab/release flicker.

Mirror the existing _wallStrafeSign A/D latch with a new _wallClimbSign:
the camera picks, per climb-key press, which of W/S climbs up (the one
pushing into the wall) via Dot(camForward, wallNormal) and latches it.
All up/down tests now run through the effective climb axis vEff -
wall->ceiling handoff, top-out detach/hop, the alongSurface mapping and
the base release - so grab and release can no longer contradict each
other. Reset on grab and in ExitWallClimb. Facing the wall and grabbing
with W is unchanged (vEff == v).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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