feat(render): Atmospheric post stack runs in linear light; neutral preset is numerically the pack-off image (Campaign VM VM3)

Closes review finding F4 (docs/research/2026-08-22-campaign-ar-review.md):
retail's main-world colour, sun rays and volumetric shafts are all
gamma-encoded display-space values (the 2013 client has no linear
lighting pipeline), but bloom thresholding, ACES (Narkowicz fit),
Rec.709 luma saturation, the contrast pivot and the vignette were all
operating directly on those gamma values, then writing the result to
the UNORM swapchain without re-encoding.

- atmospheric_common.glsl gains acdreamDecodeDisplay/acdreamEncodeDisplay
  (pow(c, 2.2) / pow(c, 1/2.2)). 2.2 is the retail-era CRT/early-LCD
  display-gamma assumption, deliberately not the sRGB piecewise curve,
  which would claim a precision retail's authoring pipeline never had.
  uAtmosphereSunColor's comment is corrected from "authored linear rgb"
  to "authored display-space rgb (retail has no linear pipeline)".
- atmospheric_bloom_downsample.frag, atmospheric_filmic.frag (both the
  fused-Low and non-fused paths) decode every world/ray/volumetric read
  before summing/thresholding; atmospheric_bloom_blur.frag is unchanged
  (it already reads the now-linear bloom buffer); atmospheric_sun_rays.frag
  and atmospheric_volumetric.frag are documented as writing display-space
  colour that the consumers decode.
- The contrast pivot moves from 0.5 (a gamma-space midpoint) to 0.18
  (linear mid-grey, the standard 18%-grey-card exposure convention).
  The final filmic output is clamped in linear, then re-encoded before
  the UNORM write.
- Bloom threshold/knee are re-derived for linear light: the pre-VM3
  gamma-space pair was threshold 1.0 / knee 0.45, i.e. a soft range of
  [0.55, 1.0] in gamma. Decoding both ends with the same 2.2 assumption
  gives decode(1.0) = 1.0 (threshold unchanged) and
  decode(0.55) = 0.55^2.2 ~= 0.27, so linear knee = 1.0 - 0.27 ~= 0.73.
  Replaced the inline 0.45f literals with named constants
  BloomThresholdLinear = 1f / BloomKneeLinear = 0.73f on
  AtmosphericPostProcessGraph. bloom-strength's 0.65 default is
  untouched.
- Exposure stays at its accepted 0.80 default: in linear,
  encode(acesFitted(0.80 * decode(0.46))) ~= 0.50, reproducing the same
  accepted midtone the old gamma-space pipeline produced as 0.51 for the
  same 0.46 input (0.46 * 0.80 fed straight into acesFitted, no
  decode/encode). Highlights now retain more (gamma 0.9 input moves from
  ~0.74 to ~0.85 through the full pipeline) and blacks deepen slightly
  (gamma 0.1 moves from ~0.09 to ~0.05) — the owner's visual gate judges.
- Added AtmosphericColorPipeline, a CPU mirror of the GLSL decode/encode/
  ACES/grade/filmic math (line-for-line, with a header comment requiring
  it stay mirrored), and AtmosphericColorPipelineTests: neutral-preset
  identity within half an 8-bit step for a 0..255 grey sweep (proving the
  neutral preset is numerically the pack-off image), decode/encode
  round-trip within 1e-6, monotonic-in-exposure, the pinned midtone/
  highlight/shadow numbers above, and the bloom-knee derivation.
- Added a shader-source pinning test so a future edit cannot silently
  drop the colour-space conversions: atmospheric_filmic.frag must
  contain exactly one acdreamEncodeDisplay( call in main()'s output,
  atmospheric_bloom_downsample.frag must contain at least three
  acdreamDecodeDisplay( calls.
- Regenerated SPIR-V (tools/compile-shaders.ps1, glslc from the
  installed Vulkan SDK). Only atmospheric_bloom_downsample.frag.spv and
  atmospheric_filmic.frag.spv changed in bytes; every other pack shader
  that includes atmospheric_common.glsl recompiled to a byte-identical
  binary (the new decode/encode helpers are unreferenced dead code for
  them). VulkanShaderManifestTests' retail-oracle SHA-256 set
  (mesh_modern, terrain_modern, mesh_detail, etc.) is untouched and
  still passes — the retail default path did not change.
- Docs: noted the linear-light move in the AR plan's Slice 1 section,
  and added a "Colour space" section to the render-pack ABI doc
  (docs/render-packs/semantic-bindings-v1.md) naming which inputs are
  display-space and pointing at atmospheric_common.glsl as the
  reference implementation. No ABI version bump — the binding layout
  is unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/plans/2026-08-21-atmospheric-rendering.md

@@ -574,6 +574,11 @@ appraisal surfaces retain their accepted colours; 1080p/1440p/4K captures show
 no clipping, haloing at the world/UI edge, stale frame, or resource leak; the
 slice meets its preset GPU/VRAM budget.
 
+Campaign VM VM3 (2026-08-22) moved the post stack to linear light: every
+world/ray/volumetric/bloom read is decoded with the 2.2 display assumption,
+tonemap/grade/vignette run in linear, and the result is re-encoded; the
+neutral preset is numerically the pack-off image.
+
 ### Slice 2 — Tier-1 screen-space sun rays
 
 **Implementation:** complete. Authored sun projection, the screen-space