Stop drawing bullets past the edge of sight -- but never sooner than is safe

Bullets were drawn everywhere on screen, including out in the black past
the edge of sight: shots that could not reach you, streaking across the
dark. They now fade out beyond sight, under one condition that bounds the
whole change: no enemy bullet may hit you without having been on screen,
at full strength, for at least WARNING_TIME (0.75s) first.

So each enemy bullet is drawn out to the edge of sight or to however far
it could close in that window with you running straight at it -- its own
speed, plus acceleration over the window, plus yours -- whichever is
further, and fades over 80 units beyond. With today's content the edge of
sight always wins: the fastest enemy bullet is 300 u/s, so head-on it is
on screen 0.83s before it lands. The speed term is what keeps that true
when something faster is added. Player shots fade at the edge of sight
however fast they are, since they cannot hurt you; a Sniper's 1500 u/s
shot was the commonest thing crossing the dark.

Shadow does not hide a bullet. One behind a wall within sight range is
still drawn over the darkness, because you can step round the corner into
it, and hiding it would be exactly the invisible hit the condition rules
out.

The guarantee is tested the hard way: every emitter in the content, and
bullets far faster than any of them, simulated head-on against a player
running at them, measuring how long each was fully drawn before it
landed. The rule's weakenings -- no speed term, forgetting the player's
own approach, ignoring acceleration -- each fail it. The connect screen's
backdrop passes no eye and draws everything, as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -98,6 +98,7 @@ against itself.
 | Prompts name the keys actually bound | done | `Settings.short_label()` |
 | Health bar: dark track, number inside, damage trail | done | `Hud._bar`, `Hud._watch_health` |
 | Centre banners: level-ups, boss phases, leaving | done | `Hud.banner`, `Hud.track_boss` |
+| Bullets fade out beyond sight, never sooner than is safe | done | `BulletRenderer.visibility()` — every enemy bullet is drawn at least 0.75s before it could land |
 | Markers pointing to party members out of sight | done | `Hud._draw_party_markers` |
 | Connect screen: live boss pattern behind it, port validation, credits | done | [src/ui/main_menu.gd](../src/ui/main_menu.gd), [src/ui/menu_backdrop.gd](../src/ui/menu_backdrop.gd) |
 | Boss bar waits until the boss has been seen | done | `Hud._draw_boss_bar`, reading the fog's `Visibility` |