feat(context): order convenience functions (#440)

[MDUYN]

Closes #440.

Summary

Adds five high-level ordering convenience functions on Context that simplify portfolio management and rebalancing. The framework previously required users to manually compute order amounts from target values/percentages — these helpers do it.

New API

Method	Behavior
order_value(symbol, value, side, price)	LIMIT order, amount = value / price
order_percent(symbol, percent, side, price)	LIMIT order for percent of portfolio net size
order_target(symbol, target_amount, price)	Rebalance to exactly target_amount units (BUY/SELL diff, no-op if equal)
order_target_value(symbol, target_value, price)	Rebalance so position value at price equals target_value
order_target_percent(symbol, target_percent, price)	Rebalance to target_percent of portfolio net size

All helpers produce LIMIT orders (require price) and validate inputs (positive value/percent/price, non-negative targets). Internally they delegate to create_limit_order; the target_* family looks up the current position and submits the BUY/SELL difference, or no order when already on target.

Example

class MyStrategy(TradingStrategy):
    def run_strategy(self, context, data):
        price = data["btc_ticker"]["price"]

        # Rebalance BTC to 10% of portfolio
        context.order_target_percent(
            target_symbol="BTC", target_percent=10.0, price=price
        )

        # Buy 500 EUR worth of ETH
        context.order_value(
            target_symbol="ETH", value=500.0,
            order_side=OrderSide.BUY, price=price
        )

        # End up with exactly 1.5 BTC
        context.order_target(
            target_symbol="BTC", target_amount=1.5, price=price
        )

Tests

14 new tests in tests/app/algorithm/test_order_convenience_functions.py covering buy paths, target-down rebalancing, no-op when already on target, and input validation errors.

============= 96 passed, 1 skipped, 2 warnings in 67.99s =============  (full tests/app/algorithm/ suite)
======================== 14 passed in 29.53s ========================= (new file only)