Add v0.3.0 changelog entry

Author: timeout187

CHANGELOG.md

@@ -3,6 +3,33 @@
 All notable changes to this project are documented here.
 Format loosely follows [Keep a Changelog](https://keepachangelog.com/).
 
+## [0.3.0] — 2026-07-10
+
+### Added
+- **General (non-axisymmetric) Euler's Equation** with the Izx cross-inertia
+  term (`src/simulator/equations_of_motion.py`, `RocketParams.Izx_initial`/
+  `Izx_final`), matching the paper's Fig. 1 "Euler's Equation" box.
+  `Izx=0` (the shipped 122mm case study's default) reproduces the prior
+  axisymmetric trajectory exactly — verified algebraically and with a
+  bit-for-bit regression test.
+- **Rotating/curved-Earth Navigation equation** (Coriolis terms,
+  R_meridian/R_normal, spherical-Earth simplification) behind the existing
+  `include_earth_rotation` flag, replacing the previous flat-NED shortcut
+  for that mode.
+- Restored pedagogical background sections (coordinate frames, equations of
+  motion, forces & moments, numerical integrator comparison) into the
+  single-page GUI as expanders.
+- `tests/test_equations_of_motion.py` — 5 new tests covering both additions.
+- GitHub Pages course site now live at
+  `https://timeout187.github.io/RocketDynamicsLab/`, cross-linked with the
+  live Streamlit app.
+
+### Fixed
+- The RK4-vs-Euler stiffness test and the new full-trajectory tests used a
+  step size (`dt=0.01`) that the exact Table 1 coefficients make unstable
+  even for RK4 over tens of seconds; switched to `dt=0.002`/`0.003`
+  matching `run_simulation()`'s own default.
+
 ## [0.2.0] — 2026-07-10
 
 ### Changed