fix: domestic current price fetching and KRX tick unit rounding (#157)

**Problem 1 — Current price always 0**
get_orderbook() used inquire-asking-price-exp-ccn which has no stck_prpr
in output1 (only askp/bidp data). This caused every domestic BUY to be
skipped with "no affordable quantity (cash=..., price=0.00)".

**Problem 2 — KRX tick unit error on limit orders**
Limit order prices were passed unrounded, triggering 호가단위 오류 in VTS.
Also ORD_DVSN was wrongly set to "01" (시장가) for limit orders.

**Fix**
- Add kr_tick_unit(price) and kr_round_down(price) module-level helpers
  implementing KRX 7-tier price tick rules (1/5/10/50/100/500/1000원).
- Add get_current_price(stock_code) → (price, change_pct, foreigner_net)
  using FHKST01010100 / inquire-price API (works in VTS, returns correct
  stck_prpr, prdy_ctrt, frgn_ntby_qty).
- Fix send_order() ORD_DVSN: "00"=지정가, "01"=시장가 (was "01"/"06").
- Apply kr_round_down() to limit order price inside send_order().
- Replace both get_orderbook() calls in main.py with get_current_price().
- Update all 4 test_main.py mock sites to use get_current_price AsyncMock.

**Tests added** (25 new tests, all 646 pass)
- TestKrTickUnit: 13 parametrized boundary cases + 7 round-down cases
- TestGetCurrentPrice: correct fields, correct API path/TR_ID, HTTP error
- TestSendOrderTickRounding: tick rounding, ORD_DVSN 00/01

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_main.py

@@ -111,15 +111,7 @@ class TestTradingCycleTelegramIntegration:
     def mock_broker(self) -> MagicMock:
         """Create mock broker."""
         broker = MagicMock()
-        broker.get_orderbook = AsyncMock(
-            return_value={
-                "output1": {
-                    "stck_prpr": "50000",
-                    "frgn_ntby_qty": "100",
-                    "prdy_ctrt": "1.23",
-                }
-            }
-        )
+        broker.get_current_price = AsyncMock(return_value=(50000.0, 1.23, 100.0))
         broker.get_balance = AsyncMock(
             return_value={
                 "output2": [
@@ -823,11 +815,7 @@ class TestScenarioEngineIntegration:
     def mock_broker(self) -> MagicMock:
         """Create mock broker with standard domestic data."""
         broker = MagicMock()
-        broker.get_orderbook = AsyncMock(
-            return_value={
-                "output1": {"stck_prpr": "50000", "frgn_ntby_qty": "100", "prdy_ctrt": "2.50"}
-            }
-        )
+        broker.get_current_price = AsyncMock(return_value=(50000.0, 2.50, 100.0))
         broker.get_balance = AsyncMock(
             return_value={
                 "output2": [
@@ -1249,9 +1237,7 @@ async def test_sell_updates_original_buy_decision_outcome() -> None:
     )
 
     broker = MagicMock()
-    broker.get_orderbook = AsyncMock(
-        return_value={"output1": {"stck_prpr": "120", "frgn_ntby_qty": "0"}}
-    )
+    broker.get_current_price = AsyncMock(return_value=(120.0, 0.0, 0.0))
     broker.get_balance = AsyncMock(
         return_value={
             "output2": [
@@ -1341,9 +1327,7 @@ async def test_hold_overridden_to_sell_when_stop_loss_triggered() -> None:
     )
 
     broker = MagicMock()
-    broker.get_orderbook = AsyncMock(
-        return_value={"output1": {"stck_prpr": "95", "frgn_ntby_qty": "0", "prdy_ctrt": "-5.0"}}
-    )
+    broker.get_current_price = AsyncMock(return_value=(95.0, -5.0, 0.0))
     broker.get_balance = AsyncMock(
         return_value={
             "output2": [