Initial travler-rotor library scaffolding

Extract Gabe Emerson's Trav'ler rotor scripts into a proper Python
library with firmware protocol abstraction (HAL 2.05 + HAL 0.0.00),
Hamlib rotctld TCP server, Click CLI, and isolated leap-frog algorithm
with the elevation copy-paste bug fixed.

src/travler_rotor/leapfrog.py

@@ -0,0 +1,50 @@
+"""Leap-frog prediction algorithm for mechanical lag compensation.
+
+The Trav'ler dish has inherent mechanical lag — by the time the motors
+reach the commanded position, a satellite in motion has already moved on.
+This module applies a small predictive overshoot so the dish "leaps ahead"
+of the target, reducing tracking error during a pass.
+"""
+
+
+def apply_leapfrog(
+    target_az: float,
+    target_el: float,
+    current_az: float,
+    current_el: float,
+) -> tuple[float, float]:
+    """Apply predictive overshoot to compensate for mechanical lag.
+
+    For each axis, if the delta exceeds a threshold, the target is nudged
+    further in the direction of travel. This keeps the dish slightly ahead
+    of fast-moving satellites.
+
+    Args:
+        target_az: Desired azimuth from tracking software (degrees).
+        target_el: Desired elevation from tracking software (degrees).
+        current_az: Last-known azimuth of the dish (degrees).
+        current_el: Last-known elevation of the dish (degrees).
+
+    Returns:
+        Adjusted (azimuth, elevation) with overshoot applied.
+
+    Note:
+        The original upstream code had a copy-paste bug where the elevation
+        delta adjustments modified target_az instead of target_el.
+        See docs/bugs.md for details.
+    """
+    # Azimuth compensation
+    az_delta = target_az - current_az
+    if abs(az_delta) > 2:
+        target_az += 1.0 if az_delta > 0 else -1.0
+    elif abs(az_delta) > 1:
+        target_az += 0.5 if az_delta > 0 else -0.5
+
+    # Elevation compensation (bug fix: original modified target_az here)
+    el_delta = target_el - current_el
+    if abs(el_delta) > 2:
+        target_el += 1.0 if el_delta > 0 else -1.0
+    elif abs(el_delta) > 1:
+        target_el += 0.5 if el_delta > 0 else -0.5
+
+    return target_az, target_el