render: add isotropic texture footprint LOD reference (#1460)

Co-Authored-By: selune <0xlillith@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/render-texture-lod.md

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+# Independent CPU LOD reference
+
+`solidcraft_render::texture_lod::select_isotropic_lod` is the small numerical
+unit in #1339, supporting the eventual wood consumer under #786. It depends only
+on existing texture admission limits and the standard library; #1327/#1332 do
+not need to merge first. It installs no texture consumer or renderer capability.
+
+Supply positive base dimensions (each at most 2048) and two derivatives of
+unwrapped UV per pixel of the actual shading grid: `duv_dx`, `duv_dy`. Every component must be finite
+and have magnitude at most 2^20. Invalid inputs refuse before calculation. The
+caller supplies perspective-correct mapping derivatives; the helper does not
+infer them from mesh/world bounds, sampled UVs or neighbouring wrapped coordinates.
+Differencing after repeat wrapping would turn a seam into a false large footprint.
+
+The shading grid is where samples are evaluated, which may differ from the final
+PNG or logical viewport grid. For Sx by Sy supersampling, derivatives measured
+per final-image pixel must be divided by Sx for `duv_dx` and Sy for `duv_dy` before
+this call. Derivatives already computed on the shading grid must not be divided
+again. Choosing a final-pixel footprint for every subsample would over-filter the
+image. Downsampling to the final image remains the consumer's separate filter.
+The helper infers no SSAA setting, device-pixel ratio or logical display scale.
+
+The texel-space Jacobian is
+`[[width*du/dx, width*du/dy], [height*dv/dx, height*dv/dy]]`.
+Its largest singular value is the footprint scale. This uses a circular screen
+differential convention, not an exact integration over the square pixel area.
+It is at least each individual column norm, accounting for shear that a maximum
+column estimate misses. An elongated footprint can over-blur its shorter axis:
+anisotropic filtering remains separate work.
+
+The implementation scales the Jacobian before forming its quadratic and uses
+f64 arithmetic. Admitted gradient/dimension bounds keep all intermediate values
+finite. Zero and subpixel footprints select LOD0. Otherwise LOD is log2(scale),
+explicitly saturated to the final level of a complete floor-halved chain ending
+at 1x1. The result includes the unsaturated scale and maximum level. For odd
+dimensions log2 remains a convention approximating non-power-of-two level ratios;
+it is not a claim of exact area integration or alias-free minification.
+
+Policy identifier `solidcraft.isotropic-singular-value-lod/1` fixes that
+convention. A consumer must explicitly reconcile it with its prepared chain;
+incomplete chains and different halving/filter policies cannot silently substitute.
+No sampling, alpha/normal interpretation, derivative persistence, allocation,
+resource cache, UI, engine command, GPU code or dependency is added. f64 reference
+results alone do not certify GPU/f32 precision or visual parity.
+
+Original analytic tests cover zero/magnification, integer and fractional LOD,
+rectangular texture scaling, rotated/signed/swapped axes, a sheared Jacobian,
+odd/single-axis chains and hostile/extreme inputs. This invisible numerical
+helper has no useful app screenshot. The visible consumer still requires typed
+bindings, local millimetre mapping, preparation ownership, CPU/GPU integration,
+PNG and offline reopen evidence. HDRI and final rendering remain separate work.