feat(holdings): REQ-043 현재 보유 평단(WAC) read — GET /v1/portfolios/{id}/holdings/cost-basis (BAT-018, PR #14)

dcy-008(confirmed)/REQ-043(Approved)/BAT-018. trex-core dcy-034 FR-7 cold-start seed 의존. AC 11/11, QC PASS. worktree+PR 워크플로 첫 파일럿.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VHtM5Ws1HKCKQ4JGi6GRb5

app/domain/pnl.py

@@ -1,4 +1,4 @@
-"""WAC (weighted average cost) based realized PnL calculation (REQ-041).
+"""WAC (weighted average cost) based realized PnL calculation (REQ-041, REQ-043).
 
 This module contains pure Python domain logic with no repository or DB dependency.
 All inputs are plain dicts or dataclasses; no database driver imports are used here.
@@ -37,6 +37,27 @@ class WacPnlResult:
     data_completeness_warning: str | None = None
 
 
+@dataclass
+class CurrentWacResult:
+    """Result of current holding WAC calculation (REQ-043).
+
+    Represents the running WAC state after replaying all ledger history,
+    giving the cost basis of currently held (unsold) shares.
+    """
+
+    current_wac: float | None
+    """WAC per unit for currently held shares. None when current_quantity is 0."""
+
+    current_quantity: float
+    """Remaining quantity after all buys and sells."""
+
+    cost_basis_from: datetime | None
+    """Earliest buy date used in WAC calculation. None when no buy history exists."""
+
+    data_completeness_warning: str | None
+    """Non-None when ledger history is incomplete (sell before buy, oversell, etc.)."""
+
+
 def calculate_realized_pnl(
     rows: list[dict[str, Any]],
     *,
@@ -164,6 +185,85 @@ def calculate_realized_pnl(
     )
 
 
+def calculate_current_wac(rows: list[dict[str, Any]]) -> CurrentWacResult:
+    """Calculate the current holding WAC (weighted average cost) from a time-ordered ledger.
+
+    Replays the full ledger history (buy/sell) and returns the running WAC state,
+    representing the cost basis of currently held (unsold) shares.
+
+    Args:
+        rows: List of portfolio_ledger row dicts, each containing:
+              side (str), quantity (float), price (float),
+              executed_at (datetime).
+              Must be sorted by executed_at ASC.
+
+    Returns:
+        CurrentWacResult with:
+          - current_wac: WAC per unit (None when current_quantity == 0)
+          - current_quantity: remaining quantity after all trades
+          - cost_basis_from: earliest buy date used
+          - data_completeness_warning: non-None when history is incomplete
+
+    WAC Rules (same as calculate_realized_pnl, REQ-041):
+        buy:  running_wac = (running_wac * running_qty + price * qty) / (running_qty + qty)
+              running_qty += qty
+        sell: running_qty -= qty (WAC per unit unchanged on sell)
+        Incomplete history (sell with running_qty=0, or sell qty > running_qty):
+              data_completeness_warning is set; calculation continues with best estimate.
+    """
+    if not rows:
+        return CurrentWacResult(
+            current_wac=None,
+            current_quantity=0.0,
+            cost_basis_from=None,
+            data_completeness_warning=None,
+        )
+
+    running_wac: float = 0.0
+    running_qty: float = 0.0
+    cost_basis_from: datetime | None = None
+    warning_triggered: bool = False
+
+    for row in rows:
+        side = str(row.get("side", "")).lower()
+        qty = float(row.get("quantity", 0.0))
+        price = float(row.get("price", 0.0))
+        executed_at: datetime | None = row.get("executed_at")
+
+        if side == "buy":
+            new_total_qty = running_qty + qty
+            if new_total_qty > 0:
+                running_wac = (running_wac * running_qty + price * qty) / new_total_qty
+            running_qty = new_total_qty
+            if cost_basis_from is None and executed_at is not None:
+                cost_basis_from = executed_at
+
+        elif side == "sell":
+            if running_qty <= 0:
+                warning_triggered = True
+            elif qty > running_qty + 1e-9:
+                warning_triggered = True
+
+            running_qty -= qty
+            if running_qty < 0:
+                running_qty = 0.0
+
+    warning: str | None = None
+    if warning_triggered:
+        warning = (
+            "이력 불완전: 보유 수량보다 많은 매도 또는 buy 이력 없는 sell이 감지되어 "
+            "cost_basis 산출이 부정확할 수 있습니다. "
+            "portfolio_ledger 이력이 포트폴리오 생성 초기부터 완전한지 확인하세요."
+        )
+
+    return CurrentWacResult(
+        current_wac=running_wac if running_qty > 0 else None,
+        current_quantity=running_qty,
+        cost_basis_from=cost_basis_from,
+        data_completeness_warning=warning,
+    )
+
+
 def _normalize_tz(dt: datetime, reference: datetime) -> datetime:
     """Align timezone-awareness of dt to match reference.