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/ROADMAP.md

@@ -84,6 +84,9 @@ play off server events. Enough to prove the pipeline, not a finished look.
 | Levels 1–15, +10 max HP each | done | [src/meta/progression.gd](../src/meta/progression.gd) |
 | XP from kills, bosses worth far more | done | `ServerRuntime._award_kill` |
 | Colour visible in world and on the HUD | done | snapshot carries it; `WorldView._draw_ship` tints |
+| Swap character from the hub | done | Esc menu → Change character; refused server-side in a dungeon |
+| XP percentage to next level | done | `HUD._draw_xp_bar` |
+| Passive health regeneration | done | `SimPlayer.regenerate`, 0.5%/s of maximum |
 
 Verified end to end by `tools/diag_progression.tscn`, which drives the real
 server through kill → xp → level → health and death → retire → roster. That
@@ -139,10 +142,10 @@ bullets spawn per shot, so they belong in the same place.
 
 | Feature | Notes |
 | --- | --- |
-| Small always-on-screen inventory | Slot count unspecified. |
+| Small always-on-screen inventory | **4 slots** for now, may grow. |
 | Health potions: rare from trash, guaranteed from bosses | |
 | World-shared loot | Player-instanced loot planned later. |
-| A unique, useless food item from bosses | Exists specifically to test player-instanced loot. Confirm whether it should be instanced *now* or just marked for it. |
+| A unique, useless food item from bosses | **Player-instanced now**, so the mechanism is exercised rather than deferred. |
 | Dropping items so others can pick them up | |
 
 ---
@@ -185,13 +188,16 @@ Not oversights — each was considered and rejected for now, with the reasoning
 
 Genuinely unspecified; do not guess at these, they change the design:
 
+Deferred to the Stage 4 discussion (upgrades), but they block that stage:
+
 1. Damage stacking order — additive pool then multiplicative, or something else?
 2. Rarity weights for the four upgrade tiers.
 3. Split shot geometry: ±22.5° from the original heading, or 45° to each side?
 4. Poison: do applications stack, or refresh a single DoT?
 5. Eraser: does it delete *enemy bullets* it passes through?
-6. Inventory slot count.
-7. Do unclaimed level-ups queue at the NPC?
-8. Does the 5-character cap count only active characters?
-9. Glass cannon's −50% health: of base HP, or of levelled max HP?
-10. What advances dungeon depth? `--depth` is a dev flag; nothing raises it in play.
+6. Do unclaimed level-ups queue at the NPC?
+7. Glass cannon's −50% health: of base HP, or of levelled max HP?
+
+Still open outside Stage 4:
+
+8. What advances dungeon depth? `--depth` is a dev flag; nothing raises it in play.