fix(galaxy): ensure spiral arms are always visible

Three fixes for circular/non-spiral galaxies produced by Randomize:

1. Raise twist range from [1.0, 6.0] to [4.0, 18.0] radians
   - Old max (~0.95 turns) could barely curve; new range covers
     0.6–2.9 full turns, matching real spiral galaxy morphology
   - Default twist raised from 3.0 to 12.0 (~1.9 turns)

2. Scale angular scatter inversely with arm count
   - Replaced hardcoded ±0.36 rad scatter with arm_scatter() that
     uses 30% of the inter-arm gap (TAU / arms × 0.30)
   - Single-arm disks use a generous 1.5 rad scatter
   - Prevents high-arm-count disks from blurring into circles

3. Bias randomize_disk toward spiral-friendly values
   - Arms weighted toward 2–4 (10%, 30%, 30%, 20%, 10% distribution)
   - Twist floored at arms × 4.0 rad so multi-arm disks always wind
     enough for distinct spiral structure
   - Outer radius defaults to "auto" 60% of the time

apps/game/src/gameplay/galaxy/mod.rs

@@ -357,7 +357,7 @@ fn generate_disk(
             let r = inner + rng.gen::<f32>().powf(0.62) * span;
             let angle = std::f32::consts::TAU * arm as f32 / arm_count as f32
                 + ((r - inner) / span) * disk.twist
-                + (rng.gen::<f32>() - 0.5) * 0.72;
+                + (rng.gen::<f32>() - 0.5) * arm_scatter(arm_count);
             let y = (rng.gen::<f32>() - 0.5) * 20.0;
             let mut candidate = Vec3::new(angle.cos() * r, y, angle.sin() * r);
 
@@ -491,6 +491,20 @@ fn generate_beam(
     }
 }
 
+/// Angular scatter width per system, scaled to the inter-arm gap.
+/// Each arm occupies `TAU / arm_count` radians; systems scatter within
+/// ~30% of that gap so arms remain visually distinct regardless of
+/// how many there are. Single-arm disks use a fixed wide scatter
+/// since there are no adjacent arms to bleed into.
+fn arm_scatter(arm_count: u32) -> f32 {
+    if arm_count <= 1 {
+        // No adjacent arms — generous scatter for a natural-looking blob.
+        1.5
+    } else {
+        std::f32::consts::TAU / arm_count as f32 * 0.30
+    }
+}
+
 /// Spacing relaxation curve — same shape as the previous single-pass
 /// generator: full spacing for the first 60 attempts, 82% until attempt
 /// 120, then 68% as a fallback to guarantee placement.

apps/game/src/gameplay/galaxy/params.rs

@@ -33,7 +33,7 @@ const DEFAULT_CORE_RADIUS: f32 = 38.0;
 // keeping the same arms/twist as the docs prototype.
 const DEFAULT_DISK_COUNT: usize = 100;
 const DEFAULT_DISK_ARMS: u32 = 4;
-const DEFAULT_DISK_TWIST: f32 = 3.0;
+const DEFAULT_DISK_TWIST: f32 = 12.0; // ~1.9 full turns
 
 // Beam defaults: small population so the two polar columns read clearly
 // without dominating the scene. Equal top/bottom for symmetry.
@@ -66,9 +66,9 @@ pub const DISK_COUNT_MAX: usize = 220;
 pub const DISK_COUNT_STEP: usize = 4;
 pub const DISK_ARMS_MIN: u32 = 1;
 pub const DISK_ARMS_MAX: u32 = 6;
-pub const DISK_TWIST_MIN: f32 = 1.0;
-pub const DISK_TWIST_MAX: f32 = 6.0;
-pub const DISK_TWIST_STEP: f32 = 0.2;
+pub const DISK_TWIST_MIN: f32 = 4.0;  // ~0.6 turns — bare minimum curvature
+pub const DISK_TWIST_MAX: f32 = 18.0; // ~2.9 turns — tight logarithmic spiral
+pub const DISK_TWIST_STEP: f32 = 0.4;
 
 /// Max tilt is 45° — disks should never be parallel to beams.
 pub const DISK_TILT_MIN: f32 = 0.0;

apps/game/src/gameplay/galaxy/ui.rs

@@ -1300,14 +1300,35 @@ fn spawn_scroll_contents_with_tabs(
 // ── Helpers ─────────────────────────────────────────────────────────────────
 
 /// Randomize all fields of a [`DiskParams`] to random valid values within
-/// their respective bounds, rounded to the UI step.
+/// their respective bounds, rounded to the UI step. Biased toward
+/// spiral-friendly combinations: arms are weighted toward 2–4, twist is
+/// floored at `arms × 4.0` radians (ensuring at least ~0.6 turns per arm),
+/// and outer radius defaults to "auto" most of the time so disks fill the
+/// galaxy.
 fn randomize_disk(rng: &mut impl rand::Rng) -> DiskParams {
     let mut disk = DiskParams::default();
     disk.count = rng.gen_range(DISK_COUNT_MIN..=DISK_COUNT_MAX);
     disk.count = (disk.count / DISK_COUNT_STEP) * DISK_COUNT_STEP;
-    disk.arms = rng.gen_range(DISK_ARMS_MIN..=DISK_ARMS_MAX);
-    disk.twist = rng.gen_range(DISK_TWIST_MIN..=DISK_TWIST_MAX);
+
+    // Weight arms toward 2–4 for clear spiral structure. 1 and 5–6 are
+    // allowed but less likely.
+    disk.arms = match rng.gen_range(0..10) {
+        0 => 1,
+        1..=3 => 2,
+        4..=6 => 3,
+        7..=8 => 4,
+        9 => rng.gen_range(5..=DISK_ARMS_MAX),
+        _ => DISK_ARMS_MIN,
+    };
+
+    // Floor twist at arms × 4.0 so multi-arm disks always wind enough to
+    // show distinct spiral structure. The floor ensures at least ~0.6 turns
+    // between adjacent arms.
+    let twist_floor = disk.arms as f32 * 4.0;
+    let effective_min = DISK_TWIST_MIN.max(twist_floor);
+    disk.twist = rng.gen_range(effective_min..=DISK_TWIST_MAX);
     disk.twist = (disk.twist / DISK_TWIST_STEP).round() * DISK_TWIST_STEP;
+
     disk.tilt = rng.gen_range(DISK_TILT_MIN..=DISK_TILT_MAX);
     disk.tilt = (disk.tilt / DISK_TILT_STEP).round() * DISK_TILT_STEP;
     disk.rotation_offset = rng.gen_range(DISK_ROTATION_MIN..=DISK_ROTATION_MAX);
@@ -1319,8 +1340,8 @@ fn randomize_disk(rng: &mut impl rand::Rng) -> DiskParams {
     disk.outer_radius = rng.gen_range(DISK_OUTER_RADIUS_MIN..=DISK_OUTER_RADIUS_MAX);
     disk.outer_radius =
         (disk.outer_radius / DISK_OUTER_RADIUS_STEP).round() * DISK_OUTER_RADIUS_STEP;
-    // Allow 0 (auto) ~50% of the time.
-    if rng.gen_bool(0.5) {
+    // Allow 0 (auto) ~60% of the time so disks fill the galaxy by default.
+    if rng.gen_bool(0.6) {
         disk.outer_radius = 0.0;
     }
     disk