evaluate_iso_20816

    Evaluate vibration severity according to ISO 20816-3 standard.

    ISO 20816-3 defines vibration severity zones for rotating machinery based on
    broadband RMS velocity measurements on non-rotating parts (bearings, housings).

    **CRITICAL - LLM Inference Policy:**
    - **NEVER infer fault type or severity from filename** (e.g., "OuterRaceFault_1.csv" does NOT mean outer race fault)
    - **NEVER assume baseline/healthy from filename** (e.g., "baseline" does NOT guarantee Zone A)
    - Treat ALL filenames as opaque identifiers
    - Report ONLY the ISO zone returned by measurement, regardless of filename
    - If filename suggests "baseline" but measurement shows Zone C/D, report Zone C/D

    **DEFAULTS** (use if user doesn't specify):
    - machine_group = 2 (medium-sized machines, most common)
    - support_type = "rigid" (horizontal machines on foundations)

    **Machine Group Selection Guide** (ask user if unsure):
    - Group 1: Large machines (power >300 kW OR shaft height H >= 315 mm)
      Examples: Large turbines, generators, compressors, large pumps
    - Group 2: Medium machines (15-300 kW OR 160mm <= H < 315mm) [DEFAULT]
      Examples: Industrial motors, fans, pumps, gearboxes

    **Support Type Selection Guide** (ask user if unsure):
    - "rigid": Machine on stiff foundation, horizontal orientation [DEFAULT]
      Rule: Lowest natural frequency > 1.25 x main excitation frequency
      Examples: Motors/pumps on concrete, horizontal compressors
    - "flexible": Machine on soft supports, vertical, or large turbine-generator sets
      Examples: Vertical pumps, machines on springs, large turbogenerators

    **When to ask user**:
    - If power/dimensions unknown -> use defaults (Group 2, rigid)
    - If clearly large turbine (>10 MW) -> suggest Group 1, flexible
    - If vertical machine -> suggest flexible
    - If user provides machine specs -> use guide above

    Evaluation Zones:
    - Zone A (Green): New machine condition - excellent
    - Zone B (Yellow): Acceptable for long-term unrestricted operation
    - Zone C (Orange): Unsatisfactory - limited operation, plan maintenance
    - Zone D (Red): Sufficient severity to cause damage - immediate action

    Args:
        signal_file: Name of the CSV file in data/signals/
        sampling_rate: Sampling frequency in Hz (default: 10000)
        machine_group: Machine group 1 (large) or 2 (medium) (default: 2 - medium)
        support_type: 'rigid' or 'flexible' (default: 'rigid')
        operating_speed_rpm: Operating speed in RPM (optional, for frequency range selection)
        signal_unit: Signal unit - 'g' or 'm/s2' (acceleration) or 'mm/s' or 'm/s' (velocity).

                     **PRIORITY ORDER FOR UNIT DETECTION:**
                     1. Check metadata file for 'signal_unit' field (recommended)
                     2. Use this parameter if explicitly provided
                     3. If neither exists: LLM will ask user to confirm based on RMS hypothesis
                     4. Default assumption: 'g' (most common for vibration sensors)

                     **IMPORTANT**: Wrong unit completely invalidates ISO 20816-3 results!
                     Best practice: Add 'signal_unit' field to metadata JSON files.

    Returns:
        ISO20816Result with evaluation zone, severity level, and recommendations

    Example:
        await evaluate_iso_20816(
            ctx,
            "motor_vibration.csv",
            sampling_rate=10000,
            machine_group=2,
            support_type="rigid",
            operating_speed_rpm=1500,
            signal_unit="g"  # Explicitly specify: 'g' or 'mm/s'
        )