feat(multiplayer): Phase 4 tasks 4.1/4.2 - local prediction history ring

Adds LocalPredictionHistory, a client-owned seq-tagged ring recording
predicted ship state per input sequence, plus wiring in
NetworkedMatch to record predictions on send and compare them against
authoritative snapshots on arrival. Ships stay frozen/interpolated
until 4.3 lands actual correction logic; this round only builds the
comparison machinery and its data.

Includes fixes from two review rounds: resync_required now
self-clears once acknowledgements catch back up (mirrors
InputJitterBuffer's stalled flag), NetBodyState gained a copy()
method to stop diagnostic accessors aliasing ring-owned state, and
corrected comments that had described the local ship as being
force-simulated pre-4.3 when it is still driven by interpolated
transform writes.

Game/scripts/net_body_state.gd

@@ -21,3 +21,27 @@ var turbo := false
 var thrust_z := 0.0          # -1..1; re-quantised to a 3-bit bin on the wire
 var stalled := false
 var avel_range := 4.0        # NetCodec.SHIP_AVEL_RANGE; set to BALL_AVEL_RANGE for the ball
+
+# Self-referential preload, not get_script().new() — this file deliberately
+# has no class_name (same cache-timing reason as test_case.gd and other
+# path-`extends`d files in this project), and get_script().new() throws
+# "Nonexistent function 'new' in base 'GDScript'" from within the script's
+# own body in this Godot version.
+const _NetBodyState = preload("res://scripts/net_body_state.gd")
+
+
+# Same contract as ShipAction.copy() (see its own comment): a distinct
+# instance with equal fields, for callers that hold onto a state past the
+# tick/comparison it was returned in.
+func copy() -> RefCounted:
+	var c := _NetBodyState.new()
+	c.position = position
+	c.rotation = rotation
+	c.linear_velocity = linear_velocity
+	c.angular_velocity = angular_velocity
+	c.frozen = frozen
+	c.turbo = turbo
+	c.thrust_z = thrust_z
+	c.stalled = stalled
+	c.avel_range = avel_range
+	return c