Bragg prediction: one limit per index, not one cube

Each Miller index is bounded by its OWN axis - |h| <= a/d_min, |k| <= b/d_min,
|l| <= c/d_min - so a single half-width has to be sized for the longest axis and
then walks the short ones far past anything the resolution cut can keep. Give the
predictor max_h, max_k and max_l instead, in all four implementations (CPU and GPU,
stills and rotation), and derive each from its own axis.

On a 149/83/226 A cell that is 23.1M candidates per frame instead of 94.2M, 4.1x
fewer. Results are bit-identical, as they must be - the candidates removed are only
ones the |q| <= 1/d_min cut rejected anyway: over six rotation crystals every merged
observation count, high-shell CC1/2 and space group matches the cube exactly, 6/6
space groups correct.

It buys almost no time, and the earlier claim that the cube cost 22% of that
crystal's wall clock was wrong. Removing 4.1x of the candidates moves it 1m58s ->
1m57s, so the whole prediction sweep is ~1% of the run. The 22% that crystal costs
relative to a fixed max_hkl of 100 is genuine extra work at max_l = 227: real
reflections inside the resolution sphere along the long axis, predicted and
integrated either way. Per-axis limits do not reduce that and cannot.

The user-facing setting stays a single number: it exists to bound the work, not to
describe the crystal, and applies to all three indices when set.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/IndexingUnitTest.cpp

@@ -264,7 +264,7 @@ TEST_CASE("PostIndexingRefinement_MultiLattice_TwoCrystals_BraggPrediction","[In
     BraggPredictionSettings pred_settings{
         .high_res_A = 2.0f,
         .ewald_dist_cutoff = 0.0010f,
-        .max_hkl = 20,
+        .max_h = 20, .max_k = 20, .max_l = 20,
         .centering = 'P',
         .wedge_deg = 0.1f,
         .mosaicity_deg = 0.2f,
@@ -364,7 +364,7 @@ TEST_CASE("FFTIndexer_MultiLattice_TwoCrystals_BraggPrediction","[Indexing]") {
     BraggPredictionSettings pred_settings{
         .high_res_A = 2.0f,
         .ewald_dist_cutoff = 0.0010f,
-        .max_hkl = 20,
+        .max_h = 20, .max_k = 20, .max_l = 20,
         .centering = 'P',
         .wedge_deg = 0.1f,
         .mosaicity_deg = 0.2f,
@@ -447,7 +447,7 @@ TEST_CASE("FFBIDXIndexer_MultiLattice_TwoCrystals_BraggPrediction","[Indexing]")
     BraggPredictionSettings pred_settings{
         .high_res_A = 2.0f,
         .ewald_dist_cutoff = 0.0010f,
-        .max_hkl = 20,
+        .max_h = 20, .max_k = 20, .max_l = 20,
         .centering = 'P',
         .wedge_deg = 0.1f,
         .mosaicity_deg = 0.2f,