Feat: Add directed asymptotic escape and analytic Schwarzschild exterior

Replace radius-only escape termination with a common asymptotic exterior protocol: declared ends, moving escape worldtubes, directed inside->outside crossings, and a PENDING_ENTRY lifecycle shared by single-frame and movie tracing.

Add an analytic Carlson-integral Schwarzschild monopole exterior (angle primitive, bracketed turning radius, ingoing Kerr-Schild coordinate-time transfer, conserved-energy frequency) so a camera outside the escape sphere is traced through an entry event.

Make the lifecycle tri-state (no ends / ready / protocol error), carry end_id through the endpoint and lens mesh, validate sources in constructors via spacetime_source_finalize(), and refresh the Schwarzschild reference images for the corrected finish.

src/spacetime_minkowski.c

@@ -33,9 +33,47 @@ static void minkowski_destroy(SpacetimeSource *source) {
   source->ops = NULL;
 }
 
+static size_t minkowski_asymptotic_end_count(const SpacetimeSource *source) {
+  (void)source;
+  return 1;
+}
+
+static int minkowski_asymptotic_end(const SpacetimeSource *source,
+                                    size_t index, SpacetimeAsymptoticEnd *out) {
+  (void)source;
+  if (index != 0)
+    return -1;
+  *out = (SpacetimeAsymptoticEnd){
+      .end_id = 0,
+      .exterior_kind = ASYMPTOTIC_EXTERIOR_MINKOWSKI,
+      .mass = 0.0,
+      .frame_origin = {0.0, 0.0, 0.0},
+      .frame_axes = {{1.0, 0.0, 0.0}, {0.0, 1.0, 0.0}, {0.0, 0.0, 1.0}}};
+  return 0;
+}
+
+static int minkowski_escape_worldtube_sample(
+    const SpacetimeSource *source, SpacetimeEndId end_id, double t,
+    SpacetimeEscapeWorldtubeSample *out) {
+  const MinkowskiContext *context = source->context;
+  if (end_id != 0)
+    return -1;
+  *out = (SpacetimeEscapeWorldtubeSample){.center = {0.0, 0.0, 0.0},
+                                          .velocity = {0.0, 0.0, 0.0},
+                                          .radius = context->escape_radius,
+                                          .radius_rate = 0.0,
+                                          .velocity_constant = 1,
+                                          .valid = 1};
+  (void)t;
+  return 0;
+}
+
 static const SpacetimeOps minkowski_ops = {
     .eval = minkowski_eval,
     .classify = minkowski_classify,
+    .asymptotic_end_count = minkowski_asymptotic_end_count,
+    .asymptotic_end = minkowski_asymptotic_end,
+    .escape_worldtube_sample = minkowski_escape_worldtube_sample,
     .destroy = minkowski_destroy,
 };
 
@@ -48,6 +86,10 @@ int spacetime_create_minkowski(SpacetimeSource *source, double escape_radius) {
   context->escape_radius = escape_radius;
   source->ops = &minkowski_ops;
   source->context = context;
+  if (spacetime_source_finalize(source)) {
+    minkowski_destroy(source);
+    return -1;
+  }
   return 0;
 }