wgpu: streaming + HiZ correctness fixes

Three correctness bugs found and fixed, plus an unrelated fly-mode
deadlock surfaced along the way.

HiZ false-rejection at the bottom of the screen
------------------------------------------------
The mip-pyramid sizing floored when halving — for a 256×70 mip 0 the
level-3 mip is 32×8, but mip-0 row 69 maps to ly = 69>>3 = 8, which is
out of bounds for an 8-row mip. ly1 then clamps down to 7 while ly0
stays at 8, the sampling loop runs zero times, max_d retains its
initial 0.0, and `min_z > 0` rejects every AABB whose projected y
range touches the bottom row. Same class for the right edge on very
wide viewports.

Fix: ceil rather than floor when halving mip dimensions so every
parent row has a covering child texel, plus std::clamp on both lookup
endpoints as belt-and-suspenders for any future mip-sizing change.

Surfaced after the user added more sidecars and saw "anything near the
bottom of the screen, no matter close or far" disappear ~0.5 s after
camera stops — that delay was the strict-VP gate + readback latency
opening the HiZ window. Found via WGPU_HIZ_TRACE rejection logs that
showed every rejection had `max_d=0` and `ly0 > ly1`.

Streaming priority lets the ocean starve out the bracing
---------------------------------------------------------
Per-instance projected screen footprint was estimated as bounding-
sphere radius squared. BIM geometry is overwhelmingly thin-in-one-axis
(slabs, pipes, columns, windows) and a flat ocean plane viewed nearly
edge-on gets a sphere projection ~250× larger than its actual screen
rect. Its chunk dominated the priority ranking and evicted the brace
chunks despite the braces being one of the closest visible things.

Fix: per-instance priority is now the screen-space AABB rectangle area
(world AABB extents projected onto the camera right/up basis vectors,
divided by view-z). Sphere radius is retained for the contribution
cull and LOD pick because conservative-over is the right failure mode
there.

Stale-VP HiZ gate
-----------------
HiZ resolves into an async ping-pong of staging buffers, so the
pyramid resident at cull time was typically captured one or two
frames ago. During camera motion the captured VP differs from
vp_this_frame and AABBs end up sampling depth taken for what was at
slightly-different screen positions in the old view. Strict by
default now: HiZ engages only when hiz_vp_ == vp_this_frame.
WGPU_HIZ_MOTION=1 trusts the stale pyramid (matches GL's default).

Fly mode Shift+Q deadlock
--------------------------
keyPressEvent requested a redraw only on the first key of a new held
set (was_empty). Pressing Shift first then Q never satisfied that
condition because Shift had already populated the set, so the render
loop never ticked. Now every relevant keypress calls requestUpdate
unconditionally.

HiZ stays opt-in behind WGPU_HIZ=1 for one release while the fix
bakes; WGPU_HIZ_TRACE=1 keeps the per-rejection diagnostic available
for future bugs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-wgpu/WgpuModelGpuData.h

@@ -210,6 +210,14 @@ struct WgpuModelGpuData {
         // with load_count >> 1 has been cycling — used by the stream
         // debug log (WGPU_STREAM_DEBUG=1) to surface thrash.
         uint32_t load_count                   = 0;
+        // Per-frame instance-aware priority. Sum of px² projected
+        // contributions of every instance owned by this chunk —
+        // captures the chunk's actual on-screen footprint, not the
+        // (often loose) AABB union projection. Computed once per
+        // frame at the top of driveStreamingLoads from the camera
+        // state; the candidate/resident priority lambdas just read
+        // this. See task #57 for the rationale.
+        float    current_priority             = 0.0f;
     };
     std::vector<Chunk> chunks;