Remove the dead legacy font fragment shader

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Background

Engine.Graphics.Vulkan.ShaderCode exports fontFragmentShaderCode, an
18-line GLSL fragment shader documented as -- | Legacy font fragment shader (non-SDF, kept for compatibility). It has no consumers:

$ rg -n 'fontFragmentShaderCode' src/ app/ test/ test-headless/ scripts/ tools/
src/Engine/Graphics/Vulkan/ShaderCode.hs:6:    , fontFragmentShaderCode
src/Engine/Graphics/Vulkan/ShaderCode.hs:69:fontFragmentShaderCode ∷ BS.ByteString
src/Engine/Graphics/Vulkan/ShaderCode.hs:70:fontFragmentShaderCode = [frag|

The module's other seven shader exports each have a real consumer:
Engine/Graphics/Font/Draw.hs for the three font shaders,
Engine/Graphics/Vulkan/Pipeline/Bindless.hs for the four bindless shaders.
The font pipeline builder hard-codes the SDF shader —
Font/Draw.hs:135, fragModule ← createShaderModule device zero { code = fontSDFFragmentShaderCode } Nothing — and both the world pipeline
(createFontPipeline, line 111) and the UI pipeline (line 324) reach it.
No branch, flag, or config can select the legacy shader.

The "compatibility" it claims is with the non-SDF font atlas path, which is
itself unreachable: every font load in the engine goes through loadSDFFont
(Engine/Scripting/Lua/Message/Texture.hs:313), the sole implementation
behind the Lua loadFont registration, which also discards the requested
size. Engine.Graphics.Font.Load.loadFont and
Engine.Graphics.Font.Atlas.generateFontAtlas have no callers.

Because [frag| … |] is a Template Haskell quasi-quote that runs glslang at
compile time, the dead shader is recompiled on every rebuild of the module.
Because the binding is exported, GHC's unused-binding warning cannot see it,
which is why -Wall -Werror has never flagged it.

Requirements

1. fontFragmentShaderCode no longer exists in
   src/Engine/Graphics/Vulkan/ShaderCode.hs — not the binding, not its
   Haddock, not its entry in the module export list.
2. The module's remaining seven shader exports are unchanged and each still
   has its existing consumer.
3. The change touches exactly one file, and the library and executable build
   warning-clean.
4. Font rendering behaviour is unchanged: the SDF fragment shader remains the
   only fragment shader either font pipeline creates.

Acceptance

rg -n 'fontFragmentShaderCode' src/ app/ test/ test-headless/   # no matches
cabal build all

Out of scope

• The unreachable non-SDF font atlas path (Font/Load.hs's loadFont,
  Font/Atlas.hs's generateFontAtlas and renderGlyphWithMetrics,
  Font/STB.hs's renderSTBGlyph, and the cbits/font_stb.c renderer behind
  them). Verified unreachable, but removing it cascades through the FFI layer
  and belongs with the dead-export cleanup (report CH-54), not here.
• The UniformBufferObject duplication across this module's other GLSL blocks
  (report CH-35, blocked on Give the bindless texture-array and handle-table sizes a single definition #975).
• Any change to SDF font rendering.

Related

• docs/code_health_findings.md CH-36 (batch 3, src/Engine/Graphics/).
• Sibling graphics dead-code removals from the same sweep: Remove the dead legacy Vulkan state types #973, Remove the never-drawn demo quad vertex buffer #974, Remove the phantom legacy path from the Vulkan texture system #977, Remove the dead bindless texture-system teardown #978.

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Automated cross-agent issue review

Reviewed against master@98e14ca1e791 by GPT-5.6-Sol.

Verdict: APPROVE

Corrections and spec additions below amend the implementation spec. Supporting context is non-normative. Open decisions remain unresolved and must not be guessed.

Spec additions / clarifications

• GPT-5.6-Sol: Acceptance must include inspection of the PR diff confirming that only src/Engine/Graphics/Vulkan/ShaderCode.hs changed and that the remaining seven shader definitions and exports are unmodified; the specified rg and build commands alone do not enforce Requirements 2 and 3.

Supporting context

• GPT-5.6-Sol: src/Engine/Graphics/Vulkan/ShaderCode.hs:6,69-86 contains the only source definition and export of fontFragmentShaderCode; repository-wide tracked-file search finds no consumer, only references in docs/code_health_findings.md describing CH-36.
• GPT-5.6-Sol: src/Engine/Graphics/Font/Draw.hs:19-21,111-135,320-324 imports the three remaining font shaders and hard-codes fontSDFFragmentShaderCode in the shared builder used by both world and UI font pipelines.
• GPT-5.6-Sol: src/Engine/Graphics/Vulkan/Pipeline/Bindless.hs:13-15,32,54 consumes all four remaining bindless shader exports, so removing the legacy binding leaves exactly seven consumed exports.
• GPT-5.6-Sol: src/Engine/Scripting/Lua/Message/Texture.hs:309-318 ignores the requested font size and calls loadSDFFont; no in-repository caller of the legacy Engine.Graphics.Font.Load.loadFont implementation exists.
• GPT-5.6-Sol: Removing the binding leaves the frag quasiquoter and qualified ByteString import in active use by the remaining fragment shaders, so the deletion does not create an unused-import warning.
• GPT-5.6-Sol: synarchy.cabal exposes the shader module but states that the package is developed from this repository and is not published to Hackage, so the verified absence of repository consumers is adequate compatibility evidence for this internal cleanup.
• GPT-5.6-Sol: No runtime rendering test is necessary for this deletion because the removed value cannot reach pipeline creation; static consumer verification plus the required library/executable build covers the affected integration surface.

Reviewer summaries

• GPT-5.6-Sol — APPROVE: The premise is verified at base SHA 98e14ca. The requested deletion is a coherent, low-risk one-file contract with sufficient build validation and no unresolved decisions.