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.

examples/issue-0029.sx

@@ -1,47 +0,0 @@
-// issue-0029: Feature — add explicit destructors to the GPU protocol so
-// resources can be freed without leaking.
-//
-// ── Proposed additions to library/modules/gpu/api.sx ──────────────────────
-//
-//   destroy_shader  :: (h: ShaderHandle);
-//   destroy_buffer  :: (h: BufferHandle);
-//   destroy_texture :: (h: TextureHandle);
-//
-// ── Why ────────────────────────────────────────────────────────────────────
-//
-// Today, library/modules/ui/glyph_cache.sx's `grow()` method recreates
-// the atlas texture at a larger size but has no way to release the old
-// one — see the comment in metal.sx that explicitly notes the leak. The
-// GL path uses glDeleteTextures(1, @self.texture_id); the GPU protocol
-// has no equivalent yet.
-//
-// ── Implementation notes ──────────────────────────────────────────────────
-//
-// Metal backend: send `release` to the MTLTexture / MTLBuffer /
-// MTLRenderPipelineState (or call CFRelease, since these are
-// CFTypeRef-compatible). Clear the corresponding slot in
-// MetalGPU.textures / buffers / shaders to `null` / 0.
-//
-// GL backend (future): glDeleteTextures / glDeleteBuffers / glDeleteProgram.
-//
-// Handle lifecycle: after destroy, the slot in the backend List is freed.
-// New allocations can take that slot or grow the list. Caller's handles
-// remain valid until destroy. Don't aggressively re-use slots in MVP;
-// keep handles append-only with a `null` marker for destroyed entries
-// (matches the current shape).
-//
-// ── Touch points ──────────────────────────────────────────────────────────
-//
-// library/modules/gpu/api.sx           — add 3 protocol method signatures
-// library/modules/gpu/metal.sx         — implement them (release + null
-//                                         the slot)
-// library/modules/ui/glyph_cache.sx    — call destroy_texture(old_handle)
-//                                         in grow() before creating the
-//                                         new atlas
-//
-// ── Syntax constraint ─────────────────────────────────────────────────────
-//
-// None — straight protocol-method addition.
-
-#import "modules/std.sx";
-main :: () -> s32 { 0; }