feat: Implement workspace system (Task #77)

Self-contained workspaces for parallel demo development.

Structure:
- workspaces/main,test - Demo-specific resources
- assets/common - Shared resources
- workspace.cfg - Configuration per workspace

CMake integration:
- DEMO_WORKSPACE option (defaults to main)
- cmake/ParseWorkspace.cmake - Config parser
- Workspace-relative asset/timeline/music paths

Migration:
- Main demo: demo.seq to workspaces/main/timeline.seq
- Test demo: test_demo.seq to workspaces/test/timeline.seq
- Common shaders: assets/common/shaders
- Workspace shaders: workspaces/*/shaders

Build:
cmake -B build -DDEMO_WORKSPACE=main
cmake -B build_test -DDEMO_WORKSPACE=test

All tests passing (36/36).

handoff(Claude): Task #77 workspace system complete. Both main and test workspaces build and pass all tests.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/workspaces/main/shaders/circle_mask_render.wgsl b/workspaces/main/shaders/circle_mask_render.wgsl
new file mode 100644
index 0000000..cfa002e
--- /dev/null
+++ b/workspaces/main/shaders/circle_mask_render.wgsl
@@ -0,0 +1,30 @@
+// Circle mask render shader
+// Samples mask and draws green outside the circle
+
+@group(0) @binding(0) var mask_tex: texture_2d<f32>;
+@group(0) @binding(1) var mask_sampler: sampler;
+
+#include "common_uniforms"
+
+@group(0) @binding(2) var<uniform> uniforms: CommonUniforms;
+
+struct VSOutput {
+  @builtin(position) position: vec4<f32>,
+};
+
+@vertex fn vs_main(@builtin(vertex_index) i: u32) -> VSOutput {
+  var pos = array<vec2<f32>, 3>(
+    vec2<f32>(-1, -1), vec2<f32>(3, -1), vec2<f32>(-1, 3));
+  return VSOutput(vec4<f32>(pos[i], 0.0, 1.0));
+}
+
+@fragment fn fs_main(@builtin(position) p: vec4<f32>) -> @location(0) vec4<f32> {
+  let uv = p.xy / uniforms.resolution;
+  let mask_value = textureSample(mask_tex, mask_sampler, uv).r;
+
+  if (mask_value > 0.5) {
+    discard;
+  }
+
+  return vec4<f32>(0.0, 1.0, 0.0, 1.0);
+}