Add closed-bar rate helpers (v0.6.0) (#26)

* Add closed-bar rate helpers and bump version to 0.6.0.

Expose drop_forming_rate_bar and multi-account collectors so downstream apps no longer need count+1 fetches and manual bar trimming.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Bump pygments to 2.20.0 to fix CVE-2026-4539 ReDoS advisory.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Address PR review feedback on closed-bar rate collection.

Validate count and start_pos before MT5 fetches, avoid redundant frame copies, clarify empty-series errors, and expand test coverage.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Include symbol and timeframe in empty closed-rate error messages.

Co-authored-by: Cursor <cursoragent@cursor.com>

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Co-authored-by: Cursor <cursoragent@cursor.com>

docs/api/sdk.md

@@ -28,6 +28,26 @@ rates = collect_latest_rates_for_accounts_with_retries(
 )
 ```
 
+### Latest closed rate bars
+
+MetaTrader 5 `start_pos=0` includes the still-forming current bar as the last
+row. `collect_latest_closed_rates_for_accounts()` fetches `count + 1` bars,
+drops that row with `drop_forming_rate_bar()`, and validates each series is
+non-empty. Use `collect_latest_closed_rates_by_granularity()` when callers
+prefer keys such as `("EURUSD", "M1")` instead of integer timeframes.
+
+```python
+from mt5cli import AccountSpec, collect_latest_closed_rates_by_granularity
+
+rates = collect_latest_closed_rates_by_granularity(
+    [AccountSpec(symbols=["EURUSD"], login=12345)],
+    ["M1", "H1"],
+    count=500,
+    retry_count=3,
+)
+closed_m1 = rates["EURUSD", "M1"]
+```
+
 ### Resolving credentials and `${ENV_VAR}` placeholders
 
 `resolve_account_spec()` / `resolve_account_specs()` merge explicit override