Debug overlay renders nothing even on the indirect drawing path

[schell]

Debug overlay renders nothing even on the indirect drawing path

Repro

1. Default headless Context (which selects the indirect/compute-culled drawing path — see the related flag-inversion issue).
2. Build a scene (e.g. the manual's shadow-mapping scene: marble bust + ground plane + directional light + shadow map).
3. stage.set_use_debug_overlay(true), render, compare to the same render without the overlay.

Result: the output is byte-identical to the non-overlay render — 0 pixels differ. No bounding-volume outlines appear.

What it is not

Instrumentation confirmed the overlay render branch is entered
(has_debug_overlay=true, maybe_indirect_buffer=Some), the bindgroup is
created, and render_pass.draw(0..6, 0..1) executes on the full-screen quad.
So the failure is inside debug_overlay_fragment
(crates/renderling/src/debug.rs): every fragment ends up writing
Vec4::ZERO.

Suspects

• The slab.read_unchecked(GeometryDescriptor::OFFSET_OF_CAMERA_ID) /
  OFFSET_OF_RESOLUTION reads returning garbage (stale stage slab buffer in the
  overlay bindgroup?)
• project_onto_viewport math producing off-screen rectangles
• The step_le(sphere_aabb.max.z, 1.0) alpha gate interacting badly with
  reversed-depth rendering, zeroing alpha for visible objects

Also worth fixing

The overlay silently no-ops when there is no indirect draw buffer (direct
drawing) — if let Some(indirect_draw_buffer) = maybe_indirect_buffer in
Stage::render. A log::warn! there would save users a debugging session.

Context

Found while writing the debug-modes manual chapter (worktree
docs-missing-manual-buildout); the chapter documents set_use_debug_overlay
with a caveat and links this issue. The overlay test step was removed from the
example because the feature currently produces no visible output.