feat: Step2~Step7 Handler 批量生成 + WorkerThread 接入新调度器

- 新增 6 个 Handler：Step2GlintDetection / Step3GlintRemoval / Step4Sampling / Step5ProcessCsv / Step6ExtractSpectra / Step7CalcIndices

- 新增 register_handlers.py：register_all_handlers() 一键注册 Step1~Step7

- 更新 __init__.py：导出全部 7 个 Handler

- 重构 worker_thread.py：移除旧 WaterQualityInversionPipeline 导入，改用 PipelineScheduler + register_all_handlers

- run_single_step 改为 scheduler.run_step() 调用，保留外部模型透传逻辑

src/core/handlers/step4_sampling.py

@@ -0,0 +1,64 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+"""
+Step4 处理器：预测采样点生成
+
+将原 WaterQualityInversionPipeline.step4_sampling() 方法
+剥离为独立的 Step4SamplingHandler。
+"""
+
+import time
+from typing import Any, Dict
+
+from src.core.handlers.base import BaseStepHandler, PipelineContext
+from src.core.steps.prediction_step import PredictionStep
+
+
+class Step4SamplingHandler(BaseStepHandler):
+    """步骤4：生成预测采样点并提取光谱。
+
+    对应 config key: 'step4_sampling'
+    委托类: PredictionStep.generate_sampling_points()
+    """
+
+    step_key = 'step4_sampling'
+
+    def execute(self, context: PipelineContext, config: dict) -> Dict[str, Any]:
+        step_start_time = time.time()
+
+        deglint_img_path = self._resolve_path(
+            config.get('deglint_img_path'), context.deglint_img_path, 'deglint_img'
+        )
+        water_mask_path = self._resolve_path(
+            config.get('water_mask_path'), context.water_mask_path, 'water_mask'
+        )
+        glint_mask_path = self._resolve_path(
+            config.get('glint_mask_path'), context.glint_mask_path, 'glint_mask'
+        )
+
+        try:
+            result = PredictionStep.generate_sampling_points(
+                deglint_img_path=deglint_img_path,
+                interval=config.get('interval', 50),
+                sample_radius=config.get('sample_radius', 5),
+                chunk_size=config.get('chunk_size', 1000),
+                water_mask_path=water_mask_path,
+                glint_mask_path=glint_mask_path,
+                output_dir=str(context.sampling_dir),
+                use_adaptive_sampling=config.get('use_adaptive_sampling', True),
+            )
+
+            step_end_time = time.time()
+            context.record_step_time(
+                "步骤4: 生成预测采样点", step_start_time, step_end_time
+            )
+
+            return {'sampling_csv_path': result}
+
+        except Exception as e:
+            step_end_time = time.time()
+            context.record_step_time(
+                "步骤4: 生成预测采样点", step_start_time, step_end_time,
+                status="failed", error=str(e)
+            )
+            raise