v0.3.0: Move g/F computation to C, eliminate Fd decision storage, overhaul visualization

C extension (_solver.c):
- Extract g-table and F-table computation from Python into C (fill_g_core,
  fill_F_core), giving ~28% speedup on these phases
- Fix critical int32 overflow in 4D stride computation: for sheets larger
  than ~180x180, the product (H0+1)*(W0+1)*(H0+1) exceeds INT_MAX.
  All strides now use int64_t
- Add defect access macros (DEF_X, DEF_Y, etc.) replacing raw
  defects[d*6+0] indexing
- Streamline defect-aligned cut loop with early x/y overlap skip

Fd memory reduction (3x):
  Previously, three 4D arrays were stored for defected rectangles:
    Fd_values  (int32) — optimal value at each (w,h,x,y)
    Fd_type    (int32) — which decision was taken (cut_x, cut_y, fill, ...)
    Fd_param   (int32) — decision parameter (cut position or item index)
  Total: 12 bytes per cell.

  Now only Fd_values (4 bytes per cell) is stored. The decision type and
  parameter are no longer needed at solve time — they are re-derived during
  reconstruction by checking which candidate cut reproduces the stored
  optimal value. This is cheap (only visited cells are reconstructed) and
  eliminates 2/3 of the Fd memory.

  For a 200x100 sheet: 201^2 * 101^2 ≈ 412M cells
    Before: 412M * 12 bytes = 4.7 GB
    After:  412M *  4 bytes = 1.6 GB

Python solver (dp_solver.py):
- All three DP phases (g, F, Fd) now call C with Python fallbacks
- Fd arrays allocated lazily (only when the sheet has defects)
- F_type array changed from int32 to int8 (decision codes fit in 3 bits)
- Reconstruction rewritten to work without stored Fd decisions:
  re-checks candidate cuts against target value to find the optimal split

Visualization (visualize.py):
- Replace TABLEAU_COLORS with a 20-color vibrant palette for better contrast
- Depth-scaling cut line widths instead of uniform 1.5pt
- Smart tick label spacing with sparse labeling to prevent overlap
- Figure width increased from 8 to 12 inches
- Dual output: SVG (vector) + PNG (raster)

Author: NicolaPerin

.vscode/c_cpp_properties.json

@@ -4,14 +4,14 @@
             "name": "Linux",
             "includePath": [
                 "${workspaceFolder}/**",
-                "/usr/include/python3.12",
-                "/home/nicola/guillotine-cutter/.venv/lib/python3.12/site-packages/numpy/core/include"
+                "/usr/include/python3.12"
             ],
             "defines": [],
-            "compilerPath": "/usr/bin/gcc",
-            "cStandard": "c11",
-            "intelliSenseMode": "linux-gcc-x64"
+            "compilerPath": "/usr/bin/clang",
+            "cStandard": "c17",
+            "cppStandard": "c++17",
+            "intelliSenseMode": "linux-clang-x64"
         }
     ],
     "version": 4
-}
+}