IPF gridded legend rendering: PUCM produces sub-pixel edge artifacts not seen in TSL/NH

# IPF gridded legend rendering: PUCM produces sub-pixel edge artifacts not seen in TSL/NH

> Draft GitHub issue body. Issues are disabled on `imikejackson/EbsdLib`;
> enable them and paste this, or file under a different repo, or convert
> to a TODO in the v3.1 plan.

## Summary

When `generate_ipf_legends` (or `LaueOps::generateIPFTriangleLegend(...,
gridded=true)`) is called with `ColorKeyKind::PUCM`, the resulting
gridded SST legends contain visible **sub-pixel white/dark dot
artifacts** along SST boundaries on most Laue classes (cubic m-3m,
cubic m-3, hex 6/m, tetragonal 4/m, trigonal -3m, trigonal -3,
monoclinic 2/m). TSL and NH gridded legends from the same code path
are artifact-free.

Sample affected outputs (after running
`Bin/generate_ipf_legends` and viewing
`Testing/Temporary/IPF_Legend//_PUCM_GRIDDED.png`):

- Cubic m-3m: ~4 single-pixel white dots along the [001]–[011] edge of
the SST, evenly spaced.
- Trigonal -3m: 2 single-pixel dark dots in the upper portion of the
kite SST.
- Tetragonal 4/m, Cubic m-3, Hex 6/m: pixel-level artifacts in the
"Discontinuous Colors" SST extensions.

## Root cause investigation (from the v3.0 release Phase 4.5 sweep)

The artifacts come from the inner PUCM key (wlenthe's
`dispatchPucm(group, direction)`) returning near-white/near-black
"singular" colors for certain `(eta, chi)` cell-center directions that
are well INSIDE the SST's angleLimits rectangle. The three existing
gridded fixes — `b3aafc4` (chi-clamp), `89aca99` (no-push-outside-SST),
`2c20533` (route 3-arg through inner key) — all live in
`GriddedColorKey` and are key-agnostic, so they're already applied to
PUCM via the shared decorator. But:

- TSL's closed-form `r = 1 − chi/chiMax` is smooth at the SST boundary;
no singularities.
- NH's `FundamentalSectorGeometry::polarCoordinates` clamps radius to
`[0, 1]` and produces graceful colors at boundaries.
- PUCM's `direction2Color(eta, chi, angleLimits)` ignores `angleLimits`
entirely (line 131 of `PUCMColorKey.cpp`: `(void)angleLimits;`),
converts `(eta, chi)` → Vec3, and hands off to wlenthe. wlenthe has
its own per-Laue-class FZ check and produces near-singular colors at
certain `(eta, chi)` cell-centers that fall just outside its
fundamental sector.

Attempted no-API-change fix (fall back to unsnapped `(eta, chi)` when
the chi clamp triggers): tested in the v3.0 prep, **did not eliminate
the visible dots**. The problematic cell-centers are inside the
angleLimits rectangle but outside wlenthe's actual SST shape
(curved [011]–[111] arc on cubic m-3m, etc.).

## Suggested fix (post-v3.0)

Add an `isInSST(const Vec3& direction)` virtual to `IColorKey` with a
`return true` default. Override in `PUCMColorKey` to call wlenthe's FZ
check (whatever PUCM uses internally — needs to be lifted to the
public API). In `GriddedColorKey::direction2Color(eta, chi, ...)`,
after snapping, reconstruct the direction and test
`m_InnerKey->isInSST(snappedDir)`. If false, fall back to the
unsnapped `(eta, chi)`.

Scope:

- ~20 lines + 1 new virtual on `IColorKey` (non-breaking; existing
inner keys outside EbsdLib compile unchanged).
- `PUCMColorKey` gets the only non-default implementation.
- TSL and NH inherit the default and are unaffected.

## Why this was deferred from v3.0

- Per-pixel IPF maps (`make_ipf`, `render_ebsd`,
`WritePoleFigureFilter`'s IPF outputs) do NOT go through
`GriddedColorKey`. They compute color per pixel directly. So the
artifacts only affect the **legend** rendering, not the IPF map.
- The artifacts are 1–2 pixels on a 1024–2000 px legend canvas — only
visible when zoomed into SST boundaries.
- The v3.0 release notes already characterize PUCM as a verbatim
wlenthe port. "wlenthe's edge behavior shows on our 1° grid" is
defensibly a wlenthe characteristic.

## References

- `Source/EbsdLib/Utilities/GriddedColorKey.cpp` — the gridded decorator
(where the eventual fix would land, plus the existing chi-clamp at
lines 107–114).
- `Source/EbsdLib/Utilities/PUCMColorKey.cpp` — PUCM's wlenthe dispatch
(line 121–135). `direction2Color(eta, chi, angleLimits)` currently
ignores `angleLimits`.
- `Source/EbsdLib/Utilities/IColorKey.hpp` — the base class that would
gain the `isInSST` virtual.
- Prior NH-era fixes: commits `b3aafc4`, `89aca99`, `2c20533`. These
apply to all inner keys but don't cover wlenthe's interior
singularities.