feat(multiplayer): Phase 3 task 3.7 - debug net overlay extension

Extends net_debug_overlay.gd (Phase 1's RTT/offset display) with the
rest of task 3.7's list: jitter (new RFC3550-style EWMA in
NetworkManager, computed from raw per-sample RTT before Phase 1's own
min-filtering, since that filter is deliberately jitter-insensitive by
design), input buffer depth and input_lead (both already tracked
client-side for task 3.3), snapshot loss (a new EWMA in networked_match.gd
over each received snapshot's own server_tick gap - snapshots go out at a
steady one-tick cadence, so a gap is direct evidence of a drop or
reorder), snapshot age (computed on demand from the same bias-corrected
tick estimate the interpolator itself uses), and bandwidth (new rolling
per-second byte counters in MatchSim, on the two 60Hz hot-path channels
only). Prediction error is deliberately omitted with a comment explaining
why: there's no client-side prediction to measure until Phase 4.

Verified values are live and plausible, not just present, by calling
get_net_debug_stats() directly in a real two-process test and checking
the numbers make sense: bandwidth matched the wire format's own byte
math almost exactly (measured ~2400 B/s sent against a computed 40B x
60Hz, ~3540 B/s received against 59B x 60Hz), and buffer depth/lead/loss
all moved in the correct direction between a clean LAN run and one under
simulated 60ms latency + 10% loss. Full regression suite re-run clean.

Game/scripts/networked_match.gd

@@ -91,6 +91,15 @@ var _input_history: Array[ShipAction] = []
 var _last_received_snapshot_tick := 0     # client only: echoed back as ack_snapshot_tick
 var _input_lead_controller := InputLeadController.new()   # client only (§3.3)
 var _last_known_input_buffer_depth := -1  # client only: -1 = no snapshot with this field yet
+# Loss estimate (task 3.7's debug overlay), client only: snapshots go out
+# at a steady one-tick cadence, so a server_tick that jumps by more than 1
+# since the last received one is direct evidence of a dropped or reordered
+# snapshot on the unreliable channel. EWMA over each reception's own
+# "missed / (missed + 1)" fraction rather than a flat drop-count, so it
+# reads as a live percentage and decays naturally once loss stops.
+const SNAPSHOT_LOSS_EWMA_ALPHA := 1.0 / 16.0
+var _snapshot_loss_ewma := 0.0
+var _expected_next_snapshot_tick := -1
 # §3.1 step 4. Not 120: InputLeadController.LEAD_MAX is 12, so anything
 # claiming to be further ahead of the current server tick than this is
 # broken or hostile, not just an honest client running a legitimately fast
@@ -405,6 +414,11 @@ func _on_snapshot_received(decoded: Dictionary) -> void:
 	var server_tick: int = decoded["server_tick"]
 	var reset_gen: int = decoded["reset_gen"]
 	var bodies: Array = decoded["bodies"]
+	if _expected_next_snapshot_tick >= 0:
+		var missed := maxi(0, server_tick - _expected_next_snapshot_tick)
+		var sample := float(missed) / float(missed + 1)
+		_snapshot_loss_ewma += (sample - _snapshot_loss_ewma) * SNAPSHOT_LOSS_EWMA_ALPHA
+	_expected_next_snapshot_tick = server_tick + 1
 	_last_received_snapshot_tick = server_tick
 	# Per-client header (§2.4): unlike the shared body segment, this is
 	# genuinely this recipient's own — input_buffer_depth is THIS client's
@@ -468,6 +482,23 @@ func _current_interp_delay_ms() -> float:
 	return clampf(one_way + SNAPSHOT_INTERVAL_MS * 1.5, INTERP_DELAY_MIN_MS, INTERP_DELAY_MAX_MS)
 
 
+# Client-only stats for task 3.7's debug overlay, discovered via the "game"
+# group the same way HUDController finds this node — no direct reference
+# needed, and the overlay degrades gracefully (has_method check) against
+# any mode that doesn't implement this at all.
+func get_net_debug_stats() -> Dictionary:
+	var snapshot_age_ms := 0.0
+	if NetworkManager.rtt_ms >= 0.0:
+		var estimated_now_tick := _estimated_tick(NetworkManager.get_server_time_estimate_ms())
+		snapshot_age_ms = (estimated_now_tick - float(_last_received_snapshot_tick)) * NetInterpolator.TICK_MS
+	return {
+		"input_buffer_depth": _last_known_input_buffer_depth,
+		"input_lead": _input_lead_controller.lead,
+		"snapshot_age_ms": snapshot_age_ms,
+		"snapshot_loss_pct": _snapshot_loss_ewma * 100.0,
+	}
+
+
 # Collider time: present-time estimate, applied once per physics tick.
 func _physics_process(_delta: float) -> void:
 	# Automatic multiplayer polling is disabled project-wide (task 1.3) —