Thunderhead - Lightning & Thunder Overhaul

# Thunderhead 1.2.0

Lightning now looks the same whether or not you use a shader pack.

### Fixed

- **Lightning is visible under shader packs.** With a pack enabled, the bolt, its branches, the wide
  glow and the air distortion were missing or wrong. The mod was asking the game to run its rendering
  programs, and a shader pack that is compositing the world has to make a decision about a program it
  does not recognise — the usual decision throws the geometry away. The mod now runs its own programs
  and applies the finished effect to the frame afterwards, so nothing has to recognise anything.
- **Everything the last release switched off under a pack is back.** The soft channel cross-section
  that makes a flat ribbon read as a volume, the atmosphere and flash passes, the heat-haze
  distortion, and ball lightning's full plasma shell, core and ground pool instead of its surface arcs
  alone.
- **A strike no longer draws a visible box on the screen.** The exposure flash had a brighter middle
  layer inset from the screen edges, and at the bright end of a flash its border showed as a
  rectangle across the frame. The flash is now a single smooth falloff.

### Unchanged, on purpose

Vanilla rendering looks exactly as it did. The new path produces the same image the old one did — the
same layers, in the same order, combined by the same rule — it just produces it somewhere the rest of
the frame cannot interfere with.

Occlusion also stays where it was: the effect is depth-tested against the depth buffer the game is
already drawing with, so terrain, water, entities and particles hide lightning exactly as before, with
a pack or without one.

### If something goes wrong

The effect degrades instead of vanishing. If a rendering program will not compile on your driver, or
the mod cannot reach the point in the frame where it applies the effect, it falls back to drawing the
older way and writes the reason to the log once. `compatibility.effectCompositor` and
`compatibility.customShaders` in `config/tempestfx.json` select those paths by hand if you ever need
them — the defaults need no attention.

Turn on `general.debug` and the overlay reports which path is live; `programs own | compositor
isolated` is the normal one.

### Tested

Minecraft 1.21.1 on NeoForge with vanilla rendering, and with Iris 1.8.12 under Complementary Unbound
r5.8.1 and ARTShade V0.3.0FIX. The same strike seed produces the same bolt in all three.