fix: prevent Mold preset wall-collapse via trail-dependent respawn

Bounce alone accumulates a wall-parallel trail that eventually condenses
all agents onto one wall-hugging filament (collapse by ~6000f). Add a
gentle trail-dependent respawn (every 90 frames, scatter agents sitting
in high-trail back into the field, probability up to 5x where brightest)
so the runaway feedback can never close. Holds a crisp large-celled vein
network out to 40000f.

Author: tamirelazar

src/preset_sim_defaults.rs

@@ -558,12 +558,19 @@ impl From<Preset> for PresetSimDefaults {
             // (moves to RenderArtDefaults). Sim levers: deposit_curve, deposit_scale,
             // decay_gamma (config.rs:744-762)
             Preset::Mold => Self {
-                // Wider sensor (22.5°) + high rotation (60°) hold a *contained*
-                // branching colony under Bounce: agents fold back at the walls into
-                // internal veins instead of condensing onto one wall-hugging filament
-                // (the old 15°/30° collapse) or wrapping toroidally (the seamless-edge
-                // character of the earlier Wrap retune). decay 0.90 keeps persistence
-                // without runaway accumulation; headless coverage holds ~0.40 @1500f.
+                // Wider sensor (22.5°) + high rotation (60°) under Bounce keep a
+                // *contained* branching colony whose veins fold back at the walls.
+                // But Bounce alone accumulates a wall-parallel trail that, given enough
+                // frames, condenses ALL agents onto one wall-hugging filament (the
+                // sensor/rotation/decay retunes only delayed it — at ~6000f the colony
+                // collapsed to a single edge filament, edge:interior trail ratio >30x).
+                // The structural fix is gentle trail-dependent respawn: every 90 frames
+                // an agent sitting in high-trail (i.e. the bright wall line) is scattered
+                // back into the field with probability rising up to 5x where the trail is
+                // brightest, so the runaway feedback can never close. Tuned as gently as
+                // possible (base 0.002) to keep the self-organised vein network intact —
+                // holds edge:interior trail ~1.1x with crisp large-celled structure out
+                // to 40000f, where decay 0.90 alone collapsed by ~6000f.
                 sensor_angle: 22.5,
                 rotation_angle: 60.0,
                 decay_factor: 0.90,
@@ -574,6 +581,12 @@ impl From<Preset> for PresetSimDefaults {
                 deposit_curve: DepositCurve::Sqrt,
                 deposit_scale: 1.5,
                 decay_gamma: 0.8,
+                respawn_config: RespawnConfig {
+                    interval: 90,
+                    base_probability: 0.002,
+                    trail_dependent: true,
+                    max_probability_multiplier: 5.0,
+                },
                 species_configs: vec![SpeciesConfig {
                     name: "default".to_string(),
                     count: 50_000,

src/profile_overrides.rs

@@ -875,6 +875,14 @@ pub(crate) fn project(ov: &ProfileOverrides) -> Result<Canonical, String> {
         sc.trail_modulation = None; // un-capturable + not runtime-editable
     }
 
+    // `respawn_config` is set by the preset layer (e.g. Mold's anti-collapse
+    // respawn).  ProfileOverrides can only carry the `interval` scalar, never the
+    // full {base_probability, trail_dependent, max_probability_multiplier}, so it
+    // cannot round-trip through capture; and no keybind mutates it at runtime.  Like
+    // `trail_modulation` above, normalise it on BOTH sides so the guard stays blind —
+    // a clean swap to a preset that sets respawn must not read dirty.
+    sim.respawn_config = Default::default();
+
     // Reproduce apply_color_aa_all priority EXACTLY ([P1]):
     //   1. full per-charset array if color_aa_all present,
     //   2. else the FULLY RESOLVED color_aa on the RESOLVED charset's slot over defaults.

tests/golden/preset_configs.txt

@@ -487,7 +487,7 @@ obstacles=[]
 attractors=[]
 separate_species_trails=false
 sampling_mode=Nearest
-respawn_config=RespawnConfig { interval: 0, base_probability: 0.01, trail_dependent: false, max_probability_multiplier: 1.0 }
+respawn_config=RespawnConfig { interval: 90, base_probability: 0.002, trail_dependent: true, max_probability_multiplier: 5.0 }
 species[0]: name="default" count=50000 sa=22.5 ra=60.0 ss=1.0 da=5.0 color=RgbColor { r: 34, g: 139, b: 34 } mod=false
 
 === etching ===