Hub character swapping, XP percentage, passive health regeneration

Character swapping is hub-only, and refused by the SERVER rather than merely
greyed out in the menu. Allowing it inside a dungeon would be an instant,
uninterruptible exit from danger -- strictly better than the one-second escape
channel, which would make that channel pointless. Creating a character in a
dungeon is refused for the same reason. Both are covered by diag_progression.

Health regenerates at 0.5% of MAXIMUM per second. A percentage rather than a
flat rate so it does not become irrelevant at level 15: a capped character
regains 1.2 hp/s against a level 1's 0.5, and both take about 200 seconds to
heal from nothing. No out-of-combat gate -- at this rate it cannot out-heal
anything actually shooting at you, and a trickle that never stops is easier to
reason about than a timer players have to learn. A fractional carry is needed
because a tick heals well under one hit point, so truncating each tick would
heal exactly nothing; there is a test for that specifically.

The XP bar now states the percentage and the level it leads to, since a bar
answers "how far" vaguely and a number answers it exactly.

Recorded the two Stage 3 answers: 4 inventory slots, and the boss's food item
is player-instanced now so the mechanism gets exercised rather than deferred.

Writing the swap guard's test caught my own mistake: the first version created
its spare character through the store, which has no opinion about where you
are, so it bypassed the guard it was meant to prove and left a stray character
behind that broke a later assertion.

200 tests. check.sh, test.sh, smoke.sh and both diagnostics pass.

src/view/game_scene.gd

@@ -20,9 +20,13 @@ func _ready() -> void:
 	get_viewport().size_changed.connect(_recentre)
 	menu.return_to_hub_requested.connect(_on_return_to_hub)
 	menu.disconnect_requested.connect(_on_disconnect)
+	menu.characters_requested.connect(func() -> void: _roster_open = true)
 	hud.respawn_pressed.connect(_on_respawn_pressed)
-	characters.select_requested.connect(func(id: String) -> void: Net.select_character(id))
+	characters.select_requested.connect(func(id: String) -> void:
+		_roster_open = false
+		Net.select_character(id))
 	characters.create_requested.connect(func(n: String) -> void: Net.create_character(n))
+	characters.closed.connect(func() -> void: _roster_open = false)
 
 
 var _screen_centre := Vector2.ZERO
@@ -60,7 +64,14 @@ func _process(_delta: float) -> void:
 	# The roster screen is shown exactly when there is nothing to play: first
 	# login, or after the last living character died.
 	if _bound != null:
-		characters.visible = _bound.needs_character() or _roster_open
+		# Forced open when there is nothing to play; opened deliberately from
+		# the menu otherwise. Never while in a dungeon -- the server refuses
+		# swaps there, so offering the screen would only invite a refusal.
+		var forced := _bound.needs_character()
+		if _bound.instance_kind != Protocol.InstanceKind.LOBBY:
+			_roster_open = false
+		characters.visible = forced or _roster_open
+		characters.set_dismissible(not forced)
 	_follow_camera()
 	menu.set_in_dungeon(_bound != null
 		and _bound.instance_kind == Protocol.InstanceKind.DUNGEON)