refactor: eliminate globals, add pen tool + edit mode, frustum culling

Remove all file-scope mutable state so the codebase compiles cleanly
as a single translation unit. State moved into structs owned by
userdata_t:

- group_index_ctx_t replaces g_group_by_id/g_group_by_id_cap
- shape_pool_ctx_t replaces g_shape_pool_dirty/g_shape_data_dirty
  and g_shape_groups/g_shape_groups_count
- panel_log_ctx_t wired through all subsystems explicitly
- pipeline_ctx_t owns render pipeline handles
- overlay_upload_state_t (per-buffer flags) replaces single bool

New features piggybacking on the refactor:

- Pen tool: click-to-place anchors, Catmull-Rom preview, finalize
  into Bezier shapes with control points
- Edit mode: anchor/handle hit testing, dragging, pre-drag undo
  snapshots, dedicated GPU buffers for edit overlays
- Frustum culling: spatial-grid-based viewport cull in draw_shapes
  with linear-scan fallback for oversized viewports
- Log dedup: FNV-1a 64-bit hash to skip duplicate messages
- Buffer shrink: halve draw buffers after 60 frames of low usage
- Shape geometry hashing for instanced-draw vertex-buffer grouping
- Group member_indices arrays with O(n) rebuild
- Log ring expanded 64→256 entries, added log_filter

Debug build: added --profiling-funcs and -sASSERTIONS flags.

makefile

@@ -65,7 +65,8 @@ debug: $(FETCH) $(SHADER_HEADERS) $(C_SOURCES) $(IMGUI_SOURCES) $(CGLM_SOURCES)
 	$(CC) $(C_SOURCES) $(IMGUI_SOURCES) $(CGLM_SOURCES) \
 		-o $(TARGET) \
 		$(EMCC_FLAGS) \
-		-g -gsource-map=inline \
+		-g --profiling-funcs -gsource-map=inline \
+		-sASSERTIONS \
 		-I$(LIB_DIR)/sokol \
 		-I$(LIB_DIR)/imgui \
 		-I$(LIB_DIR)/imgui/imgui \