test: add glyph-selection golden references; document --glyph-selection

New goldens for ascii hybrid/brightness, braille brightness, sculpted
brightness. Existing 47 goldens unchanged (mechanism default-off proof).
README gains a hybrid-glyph usage example.

Author: tamirelazar

README.md

@@ -50,6 +50,7 @@ tslime --preset zen --palette ocean --fps 24  # slow and quiet
 tslime --seed 42 --population 80000           # reproducible run, denser network
 tslime --braille --palette mono               # high-resolution monochrome
 tslime --palette-cycles 3 --palette-cycle-mode mirror  # banded contour coloring
+tslime --ascii --glyph-selection hybrid                # edge-aware directional glyphs on filaments
 tslime --export-gif demo.gif --export-frames 120 --export-fps 30
 tslime -p > frame.txt                         # print a single frame and exit
 ```

src/config_manager.rs

@@ -1959,4 +1959,53 @@ init_mode = "Random"
             "palette_cycle must equal default"
         );
     }
+
+    #[test]
+    fn missing_glyph_loads_identity() {
+        use crate::render::charset::GlyphConfig;
+
+        // Old TOML without glyph fields must deserialize and produce GlyphConfig::default().
+        let toml = r#"name = "old_no_glyph"
+population = 1000
+sensor_angle = 22.5
+sensor_distance = 9.0
+rotation_angle = 45.0
+step_size = 1.0
+decay_factor = 0.9
+deposit_amount = 5.0
+max_brightness = 100.0
+diffusion_kernel = "Mean3x3"
+diffusion_sigma = 1.0
+palette = "Organic"
+charset = "HalfBlock"
+reverse_palette = false
+invert_palette = false
+warmup_frames = 0
+food_persist = false
+auto_reset = false
+grid = false
+init_mode = "Random"
+"#;
+        let cfg: SavedConfig =
+            toml::from_str(toml).expect("old config without glyph fields must load");
+        assert!(
+            cfg.glyph_selection.is_none(),
+            "missing glyph_selection must deserialize as None"
+        );
+        assert!(cfg.glyph_edge_threshold.is_none());
+
+        // apply_to_runtime_state must produce GlyphConfig::default() (selection = None).
+        let mut state = create_test_runtime_state();
+        cfg.apply_to_runtime_state(&mut state)
+            .expect("legacy config must still apply");
+        assert_eq!(
+            state.glyph,
+            GlyphConfig::default(),
+            "missing glyph keys must default to GlyphConfig::default()"
+        );
+        assert!(
+            state.glyph.selection.is_none(),
+            "missing glyph_selection must default to None"
+        );
+    }
 }

tests/golden/ascii_brightness.txt

@@ -0,0 +1 @@
+[?2026h                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              .                                            .          ..       ..   .......  .... .  .   .....    ...                       .. .    ....   ..  ..    ......   .    ...    ..   ..  ..                          .    ....    ......  ...:...   .           ..    ....                          ..   .......     ...  ......   ..           ..   .. ..                          ..   .... .....   ..  .......  ..    ....   ..   ....                        .  .    ....        ..        ..   .    ...    .         .                          .   .....       ..       ...  ..   .....  ..   ..                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                [?2026l