gpu: destroy_shader/buffer/texture on the GPU protocol (issue-0029)

Three new method signatures on the GPU protocol. Metal backend sends
`release` to the MTLTexture/Buffer/RenderPipelineState and nulls the
slot in its backing List so the handle becomes inert; handles are not
re-used. glyph_cache.grow() now destroys the old atlas before
allocating its replacement, eliminating the per-grow leak the file's
comment had been flagging since Session 62.

library/modules/gpu/api.sx

@@ -38,6 +38,14 @@ GPU :: protocol {
     create_texture :: (w: s32, h: s32, format: TextureFormat, pixels: *void) -> TextureHandle;
     update_texture_region :: (tex: TextureHandle, x: s32, y: s32, w: s32, h: s32, pixels: *void);
 
+    // Release a GPU resource. Implementations release the backing object and
+    // null the slot so the handle becomes inert. Calling with handle 0 or
+    // an already-destroyed handle is a no-op. Handles are not re-used; the
+    // backing List entry stays at its index with a null sentinel.
+    destroy_shader  :: (sh: ShaderHandle);
+    destroy_buffer  :: (buf: BufferHandle);
+    destroy_texture :: (tex: TextureHandle);
+
     set_shader           :: (sh: ShaderHandle);
     set_vertex_buffer    :: (buf: BufferHandle);
     set_texture          :: (slot: u32, tex: TextureHandle);