v0.1.9 — Smooth screen flashes

Screen flashes now use a lightweight blended overlay in both Faithful and Modern modes, removing the full-screen palette copies, GPU readbacks, and dedicated flash shaders that could cause frame hitches.

Damage, pickup, berserk, and radiation-suit flashes retain Doom's timing and priority. Rendered colors stayed within 1/255 per channel of the original DOOM1 PLAYPAL palettes in testing.

Validation: full Go test suite, flash framebuffer tests with both modes and shader settings, and 48 Ebiten/Raylib framebuffer comparisons passed.