Doc: Record full-day parallel prebinning results and next steps

Document the producer-private adaptive path, the dummy support workload
diagnostics, the negative result of the old serial binning and the
2026-09-13 full-day comparison (splat 282.944 s vs atomic 481.417 s).
Add the corresponding raw logs, hotspot fixtures and runner evidence,
and lay out the staged plan toward a sub-240 s full-day render without
changing the default atomic path.

benchmarks/hip_psf_replay_2026-09-07/final_hip_test_adaptive.log

@@ -0,0 +1,6 @@
+COMMAND OMP_NUM_THREADS=16 OMP_DYNAMIC=FALSE env HIP_PSF_ACCUMULATION=adaptive build/Release/test_hip_psf
+TIMEOUT 45s
+GIT 2d01c2c101ccea8193a64bb46ceafca3009f0ef5
+BINARY_SHA256 385c661f266303245a0c9a6fc12bc6c6a6c50a4a7d01c4943c8abe7d048051cb
+HIP PSF cache comparison: max_abs=2.8421709430404007e-14 max_rel=7.5319184269262976e-16
+PROCESS_WALL 0.765339561 EXIT 0