Fix lobby/HUD UI pinned at (0,0); shrink player hitbox below visual size

The lobby connect menu and three pieces of the HUD (hit-flash overlay, boss
bar centering, death-message centering, hint label) were all silently broken
by the same Godot gotcha: set_anchors_preset(preset) with the default
keep_offsets=false does NOT zero the offsets to the preset's margins -- it
recomputes them to preserve the control's *current* rect, which for a
freshly constructed Control is (0,0). Anchors end up correct; the actual
rect stays pinned to the top-left corner regardless. Fixed by switching to
set_anchors_and_offsets_preset() everywhere a Control is built in code, and
by using offset_left/offset_top (anchor-relative) instead of .position
(absolute) for the HUD hint label. Documented as gotcha #5 in CLAUDE.md.

Also split PLAYER_RADIUS into two constants: PLAYER_RADIUS (6.0, the
authoritative hitbox used by SimWorld) and PLAYER_VISUAL_RADIUS (13.0,
view-only, used by world_view.gd). The client renders every ship a little
late relative to the server -- interpolation delay, reconciliation
smoothing -- so a hitbox that matched the sprite would let bullets connect
against a ship the player watched dodge clear of them. A smaller hitbox
means the occasional bullet visibly clips the sprite without a hit, which
reads as more forgiving of latency than the reverse.

Verified headlessly: a throwaway scene instantiating MainMenu/HUD under a
real 1280x720 viewport, asserting the panel is centered and _canvas.size /
hint position resolve correctly, before and after each fix. check.sh,
test.sh (78/78) and smoke.sh all pass.

src/core/sim_config.gd

@@ -19,7 +19,16 @@ const BULLET_CULL_MARGIN := 64.0
 
 # --- Player -----------------------------------------------------------------
 const PLAYER_SPEED := 240.0
-const PLAYER_RADIUS := 9.0
+## Authoritative hitbox. Smaller than PLAYER_VISUAL_RADIUS on purpose: a client
+## renders its own and everyone else's ship a little late (interpolation delay,
+## reconciliation smoothing), so a hitbox that matched the sprite exactly would
+## let bullets connect against a ship the player watched dodge clear of them.
+## Erring small means the occasional bullet visibly clips the ship without a
+## hit -- a client-side illusion, but the one that reads as fair, versus a
+## bullet that visibly missed still registering as a hit.
+const PLAYER_RADIUS := 6.0
+## Render-only. Used by src/view/, never by anything under src/sim/.
+const PLAYER_VISUAL_RADIUS := 13.0
 const PLAYER_MAX_HP := 100
 const PLAYER_FIRE_COOLDOWN := 7          # ticks
 const PLAYER_BULLET_SPEED := 620.0