docs(roadmap): record 2026-09-13 perf study and road-to-Nanite plan

docs(roadmap): record 2026-09-13 perf study and road-to-Nanite plan

Capture the Fallout 4 rendering performance study and its roadmap
in TODO.org: measured single-thread baseline (paint-dominated
571 ms frame at the Boston spawn), triangle-funnel findings
(backface/offscreen-clamp/tiny-tri waste, 3.28x overdraw, zero
frustum-culling engagement), heap/GC pressure evidence from the
live session, and the Nanite viability floor bench. Lay out the
evolutionary stages (per-cell frustum culling, per-material
backface culling, z-buffer with two-pass transparency, near-to-far
opaque order, HZB occlusion, tiny-triangle stamps, cluster-DAG
remeshing), the off-road optimization proposals, and the
exotic-technique survey with verdicts, plus the resolved design
decisions and harness notes for resuming work.

Harden the launcher alongside: install the persistent Diagnostics
log first at boot so all startup output is captured, raise the
game heap to 20 GB with heap-dump-on-OOM under
~/.cache/sixth/heapdumps, and enable the engine's 0.4 px subpixel
triangle cull by default.