Feat: Add analytic moving Alcubierre spacetime backend

Add a third analytic spacetime provider for the moving Alcubierre warp
bubble, x_s(t) = v_s*t with x_s(0) = 0.  The lab slices stay flat, so
alpha = 1, gamma_ij = delta_ij, beta^x = -v_s f(r_s), and K_ij follows
from the flat spatial metric; the time dependence enters through the
moving shape argument.  The exotic matter is treated as transparent, so
there is no capture: rays are only ACTIVE or ESCAPED, with a bubble-
centered escape radius R + 20/sigma.

Expose --alcubierre-vs, --alcubierre-radius, and --alcubierre-sigma
(|v_s| < 1).  Scale the per-ray step budget with the escape radius and
1/(1-|v_s|) so near-luminal grazing rays still escape, and reject
parameter combinations whose worst-case budget exceeds the cap.  Use a
cancellation-free shape formula for small sigma*R and reject derived
escape radii that overflow.

The regression test covers metric reconstruction, d_beta/K finite
differences, the translation isometry, small-sigma stability, the flat
limit, reflection symmetry, step convergence, and a near-luminal slow
ray.  build.md, usage.md, and README.md document the backend.

README.md

@@ -28,8 +28,9 @@ the horizon. See [single-frame camera parameters and complete commands](usage.md
 
 ## Current status
 
-The current implementation supports analytic **Minkowski** and
-**Schwarzschild** spacetimes, single images and observer-track image sequences,
+The current implementation supports analytic **Minkowski**,
+**Schwarzschild**, and moving **Alcubierre** warp-bubble spacetimes,
+single images and observer-track image sequences,
 adaptive lens meshes, and reusable lens-map files. It is written primarily in
 C with OpenMP CPU parallelism; an optional HIP backend accelerates PSF
 accumulation.
@@ -52,8 +53,8 @@ libpng development files. From the repository root:
 make -j
 ```
 
-This builds both `build/Release/minkowski_sky` and
-`build/Release/schwarzschild_sky`. For individual backends, Debug builds,
+This builds `build/Release/minkowski_sky`, `build/Release/schwarzschild_sky`,
+and `build/Release/alcubierre_sky`. For individual backends, Debug builds,
 optional HDR/FITS output, HIP support, and regression checks, see
 [build.md](build.md).