chore(training): add air_touch_bonus_weight and restart stage-5 intercepts

air_approach_weight alone didn't move productive_air_touch_fraction after a
further 180M steps (360M cumulative across all six Stage-5 attempts): an
unredirected air-intercept ball falls short of the goal from gravity and
just lands on the floor, so the already-solved ground game collects the
same episode reward whether or not anything touched the ball in the air.
air_touch_bonus_weight adds a conjunctive event bonus on top of
ball_touch_reward for a touch that's both genuinely aerial and
goal-directed, targeting the actual measured behaviour instead of only the
approach to it.

TRAINING.md

@@ -662,6 +662,34 @@ were deleted and Stage 5 restarts from Stage 4's checkpoint with the new term,
 same reasoning as every previous mechanism change: don't resume a policy
 shaped by an absent term into one where it now exists.
 
+**`air_approach_weight` alone did not fix it.** A further three attempts
+(180M more steps, 360M cumulative across all six Stage-5 attempts, 2026-08-18
+to 2026-08-19) closed with `productive_air_touch_fraction` still exactly 0.0
+and `air_touch_fraction` still at noise level, while `goal_rate` kept clearing
+its (lower) floor. Working out the actual physics instead of retuning another
+number found the real gap: an unredirected `_place_air_intercept` ball
+(spawned 6-12m up, aimed at a goal whose collision box sits at ~0-1.5m) sags
+well short of the goal from gravity alone over the required flight
+distance — it does not auto-score — so it just falls to the floor, where the
+already-solved ground game collects the exact same `goal_reward`/
+`ball_touch_reward` regardless of whether anything touched it in the air.
+Nothing in the reward ever made a genuinely aerial touch worth more than
+waiting the second or two for the ball to land, so `air_approach_weight`'s
+dense closing-speed shaping had nothing to reinforce — a second,
+independent missing-incentive gap in the same stage, not a training-duration
+problem. `air_touch_bonus_weight` (`ship_ai_controller.gd`) closes it
+directly: an event bonus on top of `ball_touch_reward`, paid only for a
+touch that is both above `AIR_TOUCH_HEIGHT` and goal-directed, scaled by the
+exact same alignment factor already gating the base touch reward —
+conjunctive, not standalone, so it targets exactly the behaviour
+`productive_air_touch_fraction` measures without reopening the RLGym
+wall-bounce exploit the original "no standalone air-touch reward" decision
+(above) was written to avoid: air-intercept/air-drill spawns are kept away
+from every wall by construction. Set to 0.5 (roughly `ball_touch_reward`'s
+own magnitude) and folded into `HANDLING_REWARD_FLAGS`. The three blocked
+attempts were deleted and Stage 5 restarts from Stage 4's checkpoint again
+with both terms active.
+
 Stage 6's `league` opponent mode samples a historical exported policy at each
 episode reset. Each later stage preserves the preceding shaping and adds one
 new difficulty.