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/ui/character_select.gd

@@ -8,12 +8,14 @@ extends CanvasLayer
 
 signal select_requested(character_id: String)
 signal create_requested(character_name: String)
+signal closed
 
 var _list: VBoxContainer
 var _name_field: LineEdit
 var _create_button: Button
 var _status: Label
 var _title: Label
+var _close_button: Button
 var _known: Array[Dictionary] = []
 var _selected: String = ""
 
@@ -63,6 +65,11 @@ func _ready() -> void:
 	_create_button.pressed.connect(_on_create)
 	row.add_child(_create_button)
 
+	_close_button = Button.new()
+	_close_button.text = "Back to the hub"
+	_close_button.pressed.connect(func() -> void: closed.emit())
+	panel.add_child(_close_button)
+
 	_status = Label.new()
 	_status.add_theme_font_size_override("font_size", 12)
 	_status.add_theme_color_override("font_color", Color(1.0, 0.6, 0.5))
@@ -144,3 +151,9 @@ func _row_for(c: Dictionary) -> Control:
 	play.pressed.connect(func() -> void: select_requested.emit(String(c["id"])))
 	row.add_child(play)
 	return row
+
+
+## Whether the player may dismiss this screen. False when they have nothing to
+## play, in which case there is nowhere to dismiss it to.
+func set_dismissible(can_close: bool) -> void:
+	_close_button.visible = can_close