feat(watchos): wire SCNTechnique post-fx via keyed-subscript uniforms (#16, #18)

[proggeramlug]

Summary

The previous `BloomPostFXTechnique` was a no-op based on the conclusion that watchOS had no path to push per-frame uniforms into an `SCNTechnique`. `handleBindingOfSymbol:usingBlock:` is genuinely unreachable (SCNRenderer isn't in the watchOS SDK), but the keyed-subscript path on the technique itself doesn't need SCNRenderer. SceneKit translates `technique[symbol] = NSData(bytes:length:)` into the matching `[[buffer(N)]]` slot whenever the symbol is declared with a `type` in the technique dictionary — this is the documented Metal uniform-binding API and works wherever SCNTechnique ships, watchOS included.

This PR:

• Builds the technique once with three `vec4` symbols mapping to `bloom_postfx.metal`'s `params0/1/2` (`[[buffer(0)]]` / `[[buffer(1)]]` / `[[buffer(2)]]` in declaration order).
• Pushes chromatic-aberration / film-grain / sun-shafts strengths each frame from the existing `PostFxState` poll.
• Defaults the sun position to top-center (`bloom_set_sun_shafts` has no positional arg, matching the issue's "above the viewport" behavior).
• Updates the surrounding architecture comments + `postfx.rs` docstring, both of which still framed Metal post-fx as deferred.

Closes #16, closes #18.

Test plan

• `cargo check --lib` in `native/watchos/` clean (no new warnings)
• `bloom_set_film_grain(0.15)` visibly overlays animated noise on the 3D helmet / fox / buggy in `examples/test-gltf-watch`
• `bloom_set_chromatic_aberration(8.0)` visibly splits R/G/B at the corners
• `bloom_set_sun_shafts(0.4, 0.85, 1.0, 0.9, 0.5)` produces amber rays from a point above the viewport
• 2D-only games unaffected (Metal pass skipped when `anyEffectActive` is false)
• Vignette + manual exposure (SwiftUI modifiers) continue to work in parallel