fix(combo): merge overlapping income weights and account costs

[Pigbibi]

Summary

• Merge overlapping core and income-layer target weights instead of overwriting shared symbols.
• Route combo proxy backtests through the existing ETF share accounting runner so no-trade drift conserves cash.
• Charge rebalance costs on actual trade deltas; cover public entrypoint and synthetic accounting regressions.

Test plan

• python3 -m unittest tests.test_combo_income_overlap tests.test_combo_simulator
• Required CI on this PR
• Consumer UES pin updates only if a material path is adopted later
• No real historical artifact recomputation in this PR

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