fix(llf,isl): guard degenerate inputs helpers

[alandtse]

Summary

Three pre-existing robustness gaps in InverseSquareLighting and ShadowCasterManager, fixed in their current inline locations. These guard degenerate / corrupt inputs that can reach the GPU lighting path or dereference garbage.

Landing this before the Tier-2 test-extraction refactor (#56) keeps that PR behavior-preserving — #56 then extracts this already-fixed code into testable headers and adds the unit tests for these exact cases.

Fixes

• InverseSquareLighting::CalculateRadius clamped only NaN. An exact-zero sqrt argument yields radius 0 (callers divide by radius, so a 0/0 attenuation fade), and a zero cutoffOverride yields +inf (sqrt(+inf) is not NaN, so the isnan-only check let it through). Now clamps any non-finite or non-positive result to 1.0.
• ShadowCasterManager::ForEachShadowLight rejected null / non-canonical / misaligned pointers but not the low 64 KiB. A near-null garbage slot (e.g. 0x8) passed the check and was dereferenced (AV/CTD). Now also rejects raw < 0x10000 (the Windows x64 null-guard region is never a valid user mapping).
• ComputeFrameTimePercentile90 guarded only count == 0; a negative count would drive a negative n into std::copy / std::nth_element (UB). Now guards <= 0.

Test plan

• Full Release (ALL, SE/AE/VR universal) build succeeds; clean link, zero /W4 /WX warnings
• Existing cpp suite passes (176 assertions / 47 cases) — no regression
• Unit coverage for these degenerate paths lands with refactor(slf): extract Tier-2 testable helpers #56 once the helpers are extracted into testable headers

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: f97ac6b0-44ea-4172-8c6f-d48bba833e63

📥 Commits

Reviewing files that changed from the base of the PR and between 38c247f and 1170b23.

📒 Files selected for processing (3)

• src/Features/InverseSquareLighting.cpp
• src/Features/LightLimitFix/ShadowCasterManager.cpp
• src/Features/LightLimitFix/ShadowCasterManager.h

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/llf-isl-degenerate-guards

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[Copilot]

Pull request overview

This PR hardens LightLimitFix and InverseSquareLighting helper paths against degenerate runtime inputs that could otherwise produce invalid GPU lighting data or unsafe shadow-light traversal.

Changes:

• Rejects near-null shadow light pointers before dereferencing.
• Handles negative frame-time sample counts with the existing fallback path.
• Clamps non-finite or non-positive inverse-square radius results to 1.0f.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/Features/LightLimitFix/ShadowCasterManager.h	Adds a low-address guard to shadow light pointer plausibility checks.
src/Features/LightLimitFix/ShadowCasterManager.cpp	Extends frame-time percentile fallback handling to non-positive counts.
src/Features/InverseSquareLighting.cpp	Ensures calculated radii are finite and positive before use by attenuation callers.

[github-actions]

No actionable suggestions for changed features.

[github-actions]

✅ A pre-release build is available for this PR:
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