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.

docs/DECISIONS.md

@@ -161,3 +161,16 @@ and then overwrite every character on the first level-up.
 
 **Account ids are written as decimal strings in JSON.** They are 64-bit and JSON
 numbers are doubles, which would silently round them.
+
+**Character swapping is hub-only, enforced on the server.** Swapping inside a
+dungeon would be an instant, uninterruptible exit from danger — strictly better
+than the one-second escape channel, and it would make that channel pointless.
+The menu greys the button out so the rule is visible, but the server refuses
+regardless of what any client's UI allows.
+
+**Health regenerates at 0.5% of MAXIMUM per second, with no out-of-combat
+gate.** 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 combat gate because 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.