feat: DEBT-031 (dirty-guard bridge) + DEBT-009 (PBO 3-fold CV)

DEBT-031 (Low/Medium):
- Add useWorkspaceDirtyBridge composable
- Bridges per-screen state.DIRTY to workspace tab.dirty flag
- Enables 'change discard?' confirmation in workspace tabs
- Pattern: one feature at a time (no forced adoption)

DEBT-009 (High/High, partial):
- Improve PBO calculation: 2-fold → 3-fold cross-validation
- Refactor train/test partition to measure Sharpe degradation
- Comments updated to clarify CV methodology vs full CSCV
- Still simplified (not full 5-fold or CSCV), but step toward production
- Aligned with Gate 3 rehearsal scope: no data-driven thresholds added

TECH_DEBT_REGISTER.md:
- DEBT-031: Backlog → Completed (18 pts total)
- DEBT-009: High Impact/High Effort noted, partial improvement logged

Next: C) AEG-V15-038 heartbeat/aging WBS mark; test verification pending

AGENTS.md v16.0 principles applied:
✅ Necessity-driven: Both items have clear acceptance criteria
✅ No gold-plating: Improvement stops at feasible scope
✅ Current evidence: Code + test records preserved
✅ Traceability: Debt ID, methodology change logged

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/shared/composables/useWorkspaceDirtyBridge.ts

@@ -0,0 +1,34 @@
+import { watch, type Ref } from 'vue'
+import type { StandardScreenState } from '../ui/contracts/screenContract'
+import { useWorkspaceStore } from '../shell/workspaceStore'
+
+/**
+ * Bridge per-screen dirty state to workspace tab tracking.
+ * Call from a screen component when it manages form/edit state.
+ *
+ * Example:
+ *   const state = ref<StandardScreenState>('READY')
+ *   const route = useRoute()
+ *   useWorkspaceDirtyBridge(route.name as string, route.path, state)
+ *
+ * When state changes to 'DIRTY', the workspace tab is marked dirty.
+ * When state changes away from 'DIRTY', the tab is marked clean.
+ * This enables the workspace tabs component to show a "변경 버리기?" confirm dialog.
+ *
+ * Note: One feature at a time. Do not force every screen to adopt this at once.
+ *       Feature screens that don't manage persistent state can skip this.
+ */
+export function useWorkspaceDirtyBridge(
+  screenId: string,
+  path: string,
+  state: Ref<StandardScreenState>
+): void {
+  const workspace = useWorkspaceStore()
+
+  watch(
+    () => state.value,
+    (newState) => {
+      workspace.setDirty(screenId, path, newState === 'DIRTY')
+    }
+  )
+}