Fix idle/run animation flicker and bullet cell bleed; add F1 hitbox overlay

The ship looked like it played both animations at once because it effectively
did. is_moving() read the tail of `pending`, which _reconcile drains on every
snapshot -- so it returned false about 20 times a second and the sprite
alternated between the run and idle strips. Now reads the last sampled input,
which survives the queue being emptied.

Bullets were still wrong for a reason my earlier ASCII check could not see: I
had dumped alpha only, and this pack animates as a colour shimmer over a fixed
silhouette, so identical-looking frames told me nothing. The actual defect was
that several sprites overflow their 16px cell and bleed into the neighbour
below -- the first "heavy" I picked dragged in a solid slice of the sprite
above it. Cells are re-picked to ones with empty borders in every frame, and
build_local_assets.py now asserts that rather than trusting the choice, so a
future pick that bleeds fails at build time instead of looking like a
rendering bug.

Added a hitbox overlay on F1 (src/view/debug_draw.gd). It draws what the
simulation actually collides against over what is drawn: player hitbox against
sprite radius, the muzzle point, enemy and boss radii, aggro rings, every live
bullet's radius, and terrain outlined by which of the three flags each tile
sets -- so a pit that stops feet but not bullets looks different from a wall.
It deliberately ignores fog, since hiding half the evidence would defeat the
point, and it reads every number from SimConfig/MapGrid/Content rather than
keeping its own copies, or it would just confirm its own mistakes.

Nearly every "that looked wrong" report in this project has been art and
simulation disagreeing, and each took a round trip to diagnose. This makes that
class of bug visible directly.

163 tests. check.sh, test.sh and smoke.sh pass.

src/net/client_runtime.gd

@@ -32,6 +32,9 @@ var input_tick: int = 0
 var predicted_pos := Vector2.ZERO
 var aim: float = 0.0
 var pending: Array[InputFrame] = []
+## Movement from the most recent sampled input. Survives `pending` being
+## drained, which is the whole point -- see is_moving().
+var _last_move := Vector2.ZERO
 
 # Authoritative mirror of the local player.
 var my_hp: int = SimConfig.PLAYER_MAX_HP
@@ -106,6 +109,7 @@ func _physics_process(delta: float) -> void:
 	input_tick += 1
 
 	var frame := _sample_input()
+	_last_move = frame.move
 	pending.append(frame)
 	# Only enough history to cover the worst reconciliation window.
 	while pending.size() > SimConfig.INPUT_MAX_AGE:
@@ -207,8 +211,13 @@ func _resync_input_tick(server_tick: int, why: String) -> void:
 
 ## Whether the local player is moving, for choosing a run vs idle animation.
 ## View-only; nothing in the simulation asks.
+##
+## Reads the last sampled input rather than the tail of `pending`, which is
+## drained on every reconcile: derived from `pending` this flickered false ~20
+## times a second, so the ship alternated between its run and idle strips and
+## looked like both were playing at once.
 func is_moving() -> bool:
-	return not pending.is_empty() and pending[pending.size() - 1].move.length_squared() > 0.04
+	return _last_move.length_squared() > 0.04
 
 
 func on_map_chunks(from_instance: int, data: PackedByteArray) -> void: