RustyNES v2.2.8 — "Aperture II" (gamma-aware scanlines + sharper CRT)

A presentation-fidelity release addressing the NESdev-forum feedback on
gamma-aware resampling and bilinear-soft scanlines. Presentation-only — nothing
here touches the emulation core, so the pre-shader framebuffer, save-states, and
every golden vector are byte-identical (AccuracyCoin 141/141, nestest 0-diff), and
the shipped native default is byte-identical to v2.2.7 (the native sRGB
surface passes aux = 0, which selects the exact pre-v2.2.8 scanline profile;
the new linear-light + sharper-scanline path activates only for a non-zero
aux, set on the WebGL2 non-sRGB path and when the scanline knob is raised).
The base BLEP audio and the advanced CRT stacks
(royale/guest/megatron, already gamma-correct) are untouched.

Visual verification pending. These are shader/appearance changes; naga
validates that the WGSL compiles and the native/wasm builds are clean, but the
on-screen result must be confirmed on a real display + a browser (WebGL2).

Changed

• Gamma-correct scanlines + aperture mask (base CRT pass, CRT_WGSL). The
  scanline/mask darkening now happens in linear light. On the native path
  (sRGB texture + surface) the sampler/surface already convert, so the shader
  leaves it linear (aux.y = 0, output byte-identical). On a plain UNORM path
  (WebGL2, which does neither) the shader now sRGB-decodes on read and
  re-encodes before output (aux.y = 1) — fixing a real browser-only gamma bug so
  a scanline valley is 50% of the linear luminance, not the encoded value. The
  round-trip uses the exact IEC 61966-2-1 piecewise sRGB transfer (the
  0.04045 / 0.0031308 breakpoints + a 2.4 exponent), not a pow(2.2)
  approximation, so the WebGL2 result matches the hardware sRGB surface the native
  path uses bit-for-bit.
• Sharper scanlines (aux.x, default 0.5). The scanline profile blends from
  the original soft parabola (0) to a narrow Gaussian beam (1) for crisp vertical
  boundaries instead of the linear-sampler blur — the sharper scanlines the
  feedback asked for. Only visible when scanlines are enabled; aux.x = 0
  reproduces the pre-v2.2.8 profile exactly. Wired on the desktop
  (rustynes-frontend), Android (rustynes-android), and iOS Metal
  (rustynes-ios) hosts via the shared 16-float CRT uniform
  (rect + crop + params + aux) — all three set aux identically for the
  scanline/CRT filters, so the corrected profile is consistent across platforms.