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.

CLAUDE.md

@@ -87,7 +87,16 @@ ticks in milliseconds with no SceneTree.
 4. **Input events default to `device = 16`, which matches nothing.** Bindings
    must use `device = -1`. `tools/setup_input_map.gd` generates the input map
    correctly; edit that file, not the `[input]` block in `project.godot`.
-5. **`ProjectSettings.save()` drops settings equal to the engine default** and
+5. **`set_anchors_preset(preset)` does not zero the offsets.** `keep_offsets`
+   defaults to `false`, which despite the name means "recompute offsets to keep
+   the control's *current* rect on screen" — for a freshly created Control that
+   rect is `(0,0)`-sized, so it comes out pinned to the top-left corner
+   regardless of the anchors. Use `set_anchors_and_offsets_preset()` for any
+   Control built in code. Separately: `.position` assigns an *absolute*
+   coordinate even on an anchored control; `offset_left`/`offset_top` are the
+   anchor-relative ones. This combination silently broke the main menu and
+   three pieces of the HUD.
+6. **`ProjectSettings.save()` drops settings equal to the engine default** and
    strips comments. Anything load-bearing (the 60 Hz tick) is asserted in code
    in `src/main.gd` instead of trusted to `project.godot`.