perf(step11): IDW auto-switch for large grids + GDAL fast rasterize

- 网格 >500K 点自动跳过 Kriging 切 IDW（千万级点从数小时降至数秒）
- n_closest_points 50→20，Block 每块打印进度
- GDAL ReprojectImage 直接重采样栅格掩膜，避免 58K 多边形 rasterize 卡死
- 自动扫描工作目录 .dat 栅格，不依赖 handler 传参
- 特殊 Unicode 字符替换为 ASCII 兼容

src/core/handlers/step11_map_handler.py

@@ -193,11 +193,42 @@ class Step11MapHandler(BaseStepHandler):
                     input_crs=base_kwargs['input_crs'],
                     output_crs=base_kwargs['output_crs'],
                 )
+                # 原始路径若是栅格（.dat/.tif），传给 prepare_shared_context
+                # 走 GDAL 快速重采样通道，避免 58K 多边形 rasterize 卡死
+                _orig_path = boundary_shp_path
+                _raster_src = None
+                _RASTER_EXTS = ('.dat', '.tif', '.tiff', '.bsq', '.bil', '.bip', '.img')
+                if _orig_path and os.path.isfile(_orig_path):
+                    _ext = os.path.splitext(_orig_path)[1].lower()
+                    if _ext in _RASTER_EXTS:
+                        _raster_src = _orig_path
+                # fallback: 扫描 work_dir 下的原始栅格掩膜
+                if _raster_src is None:
+                    _wd = Path(str(context.work_dir))
+                    for _sub in ('1_water_mask', '1_Water_Mask', 'water_mask'):
+                        _scan_dir = _wd / _sub
+                        if _scan_dir.is_dir():
+                            for _ext in _RASTER_EXTS:
+                                _hits = sorted(_scan_dir.glob(f'*{_ext}'),
+                                               key=lambda p: p.stat().st_mtime, reverse=True)
+                                for _h in _hits:
+                                    if _h.is_file():
+                                        _raster_src = str(_h)
+                                        break
+                                if _raster_src:
+                                    break
+                        if _raster_src:
+                            break
+                if _raster_src:
+                    context.notify('step11_map', 'info',
+                                  f'找到原始栅格掩膜，将使用 GDAL 快速重采样: {Path(_raster_src).name}')
+
                 shared_ctx = pre_mapper.prepare_shared_context(
                     sample_csv=csv_paths[0],
                     shp_file=resolved_boundary,
                     resolution=float(base_kwargs['resolution']),
                     expand_ratio=0.05,
+                    boundary_raster=_raster_src,
                 )
                 base_kwargs['shared_context'] = shared_ctx
                 context.notify('step11_map', 'info',