bug(streaming-ui): sub-cent costs show raw Decimal arithmetic precision (e.g. $0.00711955 instead of $0.0071)

Repo: amplifier-module-hooks-streaming-ui
File: amplifier_module_hooks_streaming_ui/__init__.py — format_cost_usd()

Evidence

Real haiku session output:

└ Input: 12,530 (64% cached) | Output: 145 | Total: 12,675 | Cost: $0.00711955

Expected: $0.0071 (2 sig figs). Got: $0.00711955 (8 digits, 7 decimal places).

Root cause

format_cost_usd used input_exp = -cost.as_tuple().exponent to determine decimal places. This reads stored Decimal arithmetic precision, not display precision. When compute_cost multiplies token counts by rate constants like Decimal("3.000"), Python stores the full intermediate precision (8–12+ places), making input_exp far larger than needed.

Fix (already applied on feat/m0-cost-management)

# Remove input_exp entirely. Use exp_floor for 2 sig figs + strip trailing zeros.
decimal_places = -exp_floor + 1
result = f"${cost:.{decimal_places}f}"
return result.rstrip("0") or "$0.00"

Open amplifier_module_hooks_streaming_ui/init.py and inspect format_cost_usd(), then reproduce the sub-cent output shown in the issue. Compare the function with the fix already applied on feat/m0-cost-management. Done means sub-cent costs display with two significant figures, such as $0.0071, without raw Decimal precision.