docs(multiplayer): consolidate tracking into one document

multiplayer-todo.md and multiplayer-next.md tracked overlapping
information in two places. Fold everything into multiplayer-next.md
(architecture decisions, wire format, task breakdown with checkboxes,
gotchas list, testing notes) and delete multiplayer-todo.md. Section
numbers are unchanged, so existing code comments citing them by
section/task number still resolve; update every such reference to
point at the new filename.

Game/scripts/video_settings.gd

@@ -24,7 +24,7 @@ extends Node
 # independently of stretch mode, since it scales the 3D viewport's own internal
 # resolution before this blit rather than the window itself. Task 0.15b also
 # found an unexplained ~6% non-uniform width scaling on this project's one
-# tested (Mac/Retina) machine — see multiplayer-todo.md §5.5.1 — which needs
+# tested (Mac/Retina) machine — see multiplayer-next.md §5.5.1 — which needs
 # understanding before stretch mode is touched, not blindly carrying into a
 # resolution-dependent change.
 #
@@ -49,7 +49,7 @@ const SETTINGS_PATH := "user://settings.cfg"
 # preset -> bundle applied to the individual fields below. CUSTOM has no
 # bundle: selecting it just stops future preset changes from overwriting
 # whatever the individual fields currently hold. Task 0.15b's measured
-# per-effect costs (multiplayer-todo.md §5.5.1) were too noisy to rank these
+# per-effect costs (multiplayer-next.md §5.5.1) were too noisy to rank these
 # against each other, so each rung is "meaningfully fewer full-screen passes
 # than the one above it" rather than a precisely tuned ladder.
 const PRESET_BUNDLES := {
@@ -78,7 +78,7 @@ var shadows_enabled: bool = true
 var glow_enabled: bool = true
 
 # FXAA alone, not MSAA_FXAA: 4x MSAA *and* FXAA stacked is redundant blur for
-# most scenes and costs more than either alone (see multiplayer-todo.md 0.19).
+# most scenes and costs more than either alone (see multiplayer-next.md 0.19).
 var aa_mode: AAMode = AAMode.FXAA
 var glow_scale: float = 1.0
 var brightness: float = 1.0
@@ -252,7 +252,7 @@ func apply_fps_cap() -> void:
 # Called once by each arena's _ready() (and again on settings_changed, so an
 # already-loaded arena updates live) to fold the user's glow/brightness
 # preference into that arena's own baked Environment tuning, and to gate the
-# preset-controlled full-screen passes (§5.5 of multiplayer-todo.md).
+# preset-controlled full-screen passes (§5.5 of multiplayer-next.md).
 func apply_to_environment(env: Environment) -> void:
 	if env == null:
 		return