v0.2.0: splats, copy/paste, faster homes, seat facing, and a full bug sweep

Frame Studio
- Gaussian splats (PLY, SPZ v2-v4), PLY vertex colours, panorama skies
- Copy, cut, paste and duplicate for objects, splats and world items
- Walkable-area rebake moved off the UI thread; viewport one frame sooner;
  refresh-locked pacing (FS_PACE=timer for the old one)
- Fixed: bundle thumbnail use-after-free crash, auto-save deleting
  environments, saves into scratch copies, edits lost on non-ASCII paths,
  unopenable manifests with inf/nan, bundle save data loss, opening a home
  marking it changed, gizmo NaN, long paths, many malformed-file crashes
- Updater verifies sha256, journals the swap and never leaves the exe missing

Frame Home
- Seats turn you to face their way
- Draw batching (Hand Tutorials up to ~20% less GPU time), cut-out EQUAL pass,
  late head-pose read before submit
- Proper queue-family hand-over to the compositor layer; layer descriptor
  generations and seqlocked slots; structural SPIR-V check
- Fixed: hangs/crashes on damaged .fenv files and shaders, render-pass
  validation error, sync hazard in the foveation smoothing pass, unverified
  update downloads, half-done installs

Tests: tests_loco/env_share/spv_check wired into CMake (FRAME_TESTS) and CI.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VsaXGU27gsrXjDoFtyFJP9

third_party/spirv-tools/source/to_string.cpp

@@ -0,0 +1,44 @@
+// Copyright (c) 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "source/to_string.h"
+
+#include <cassert>
+
+namespace spvtools {
+
+std::string to_string(uint32_t n) {
+  // This implementation avoids using standard library features that access
+  // the locale.  Using the locale requires taking a mutex which causes
+  // annoying serialization.
+
+  constexpr int max_digits = 10;  // max uint has 10 digits
+  // Contains the resulting digits, with least significant digit in the last
+  // entry.
+  char buf[max_digits];
+  int write_index = max_digits - 1;
+  if (n == 0) {
+    buf[write_index] = '0';
+  } else {
+    while (n > 0) {
+      int units = n % 10;
+      buf[write_index--] = "0123456789"[units];
+      n = (n - units) / 10;
+    }
+    write_index++;
+  }
+  assert(write_index >= 0);
+  return std::string(buf + write_index, max_digits - write_index);
+}
+}  // namespace spvtools