fix(backtest): correct internal combo weight rounding overflow

[Pigbibi]

Problem and change

Valid ETF/dividend allocations can produce a combined target sum of 1.0000000000000002 after floating-point arithmetic, causing lifecycle preflight to reject the portfolio. Correct only a one-ULP overflow introduced internally by subtracting the excess from the largest target; the final sum must still be at most one.

Reject invalid or over-budget leg allocations before combination. Preserve rejection of negative, non-finite, and over-budget signal weights.

Validation

• Focused regression suite: 44 passed, 7 subtests passed.
• Tests cover the original rounding reproduction, strict final sum, zero returns with constant prices and zero costs, and rejection of a configured budget even one ULP above one.
• Ruff and git diff --check passed.
• No real-data backtest or production workflow rerun performed; this PR does not establish remote Drift Check recovery.