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/hud.gd

@@ -156,10 +156,17 @@ func _draw_xp_bar(at: Vector2) -> void:
 	if who.is_empty():
 		return
 	var level := int(who["level"])
-	var progress: float = who["progress"] if level < Progression.MAX_LEVEL else 1.0
-	_canvas.draw_rect(Rect2(at, Vector2(BAR_W, 4.0)), Color(0.1, 0.12, 0.18))
-	_canvas.draw_rect(Rect2(at, Vector2(BAR_W * progress, 4.0)),
-		Color(0.6, 0.55, 1.0) if level < Progression.MAX_LEVEL else Color(1.0, 0.85, 0.4))
+	var capped := level >= Progression.MAX_LEVEL
+	var progress: float = 1.0 if capped else who["progress"]
+	var tint := Color(1.0, 0.85, 0.4) if capped else Color(0.6, 0.55, 1.0)
+	_canvas.draw_rect(Rect2(at, Vector2(BAR_W, 5.0)), Color(0.1, 0.12, 0.18))
+	_canvas.draw_rect(Rect2(at, Vector2(BAR_W * progress, 5.0)), tint)
+	# The number as well as the bar: "how far to the next level" is a question
+	# a bar answers vaguely and a percentage answers exactly.
+	var text := "MAX" if capped else "%d%% to level %d" % [
+		int(floor(progress * 100.0)), level + 1]
+	_canvas.draw_string(ThemeDB.fallback_font, at + Vector2(BAR_W + 10.0, 6.0),
+		text, HORIZONTAL_ALIGNMENT_LEFT, -1, 12, tint)
 
 
 ## Shown after the boss dies, so the victory lap has a visible clock on it.