feat: Add per-coin order size overrides

[keitaj]

Summary

• Add --coin-size-overrides CLI flag to configure ORDER_SIZE_USD per coin (e.g., "TSLA:150,NVDA:150")
• Coins without overrides fall back to the global --order-size-usd value
• Setting a coin's size to 0 skips order placement for that coin
• Follows the same lookup pattern as existing --coin-offset-overrides and --coin-spread-overrides (full name -> bare name -> global default)

Changes

• strategies/market_making_strategy.py: Add _coin_size_overrides dict, _get_coin_size() helper, and update calculate_position_size() to use per-coin lookup
• bot.py: Add --coin-size-overrides argparse flag and wire it into _STRATEGY_PARAMS
• README.md: Document the new parameter in strategy description and YAML reference
• tests/test_per_coin_order_size.py: 13 test cases covering override lookup, bare/prefixed name matching, zero-value skip, global fallback, and risk_level multiplier interaction
• 8 existing test files: Add _coin_size_overrides = {} to test helpers that bypass __init__

Test plan

• All 766 tests pass (pytest tests/ -q)
• Lint passes (flake8 --max-line-length=120)
• Verify --coin-size-overrides "TSLA:150" reduces TSLA order size while other coins use global default
• Verify --coin-size-overrides "TSLA:0" skips TSLA orders entirely
• Verify backward compatibility: omitting the flag keeps existing behavior

🤖 Generated with Claude Code

[keitaj]

Review: Per-coin order size overrides

Clean implementation following the established _get_coin_*() pattern. 13 tests with real assertions.

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1. Existing test helpers need _coin_size_overrides (informational)

8 existing test files updated to add _coin_size_overrides = {} — this is correct and necessary because those tests bypass __init__ and manually set attributes. ✓

2. _get_coin_size follows exact same pattern (informational)

Identical to _get_coin_offset() and _get_coin_spread(): full name → bare name → global default. Consistent. ✓

3. inventory_skew still uses global order_size_usd (informational)

_calculate_inventory_skew() normalizes position value by self.order_size_usd, not the per-coin size. This is correct — skew normalization is about the account-level position scale, not the per-coin order size. ✓

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Verdict: LGTM

• No blockers or should-fix items
• 13 new tests covering all lookup scenarios, fallback, zero value, risk multiplier
• Fully backward compatible (empty string default)
• Clean reuse of existing _parse_coin_overrides() parser
• README updated for both human and AI reference

Merging after CI check.