docs(design): Sol reads the code -- three changes worth testing, one determinism hole

Round two of the owner's "build it every better way" question: Sol's round-one session was forked
read-only and given the repository, to say what could reasonably change and how each change would
prove its worth. Advice only; nothing was changed, built or run.

Sol's verdict: don't rebuild. The representation, mesher, clipmap and operator stack are already
close to its clean-slate design. Three things are worth testing, each with a cheap
instrumentation-only first step and explicit kill criteria:
- tile-wide, feature-major tunnel evaluation (inferred 8-18% of worker CPU);
- exact density reuse across adjacent gameplay tiles (theoretical ceiling ~23%);
- a canonical numeric contract for world-deciding maths (correctness, ~neutral on speed).

The orchestrator verified against the code and the UE 5.7 engine source:
- FMath::Sin/Cos resolve to the CRT sinf/cosf on Win64 (FMA3 variant chosen at runtime);
- per-sample lookup is ~5% exclusive, so round one's "the loop direction is the largest gap"
  overstated it;
- worker-thread parse-once flags, raw-memory CRC cache keys and a docs-vs-code contradiction on
  landing connectors are real.
Round one's note gets a pointer to the corrections.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TaFdXhTdYMkW1xrV6ApT6Y

DESIGN-SOL-2026-09-15.md

@@ -576,3 +576,5 @@ GPU occlusion and meshlet culling help rendering, but avoiding CPU generation of
    Predict reachable motion including falls, prefetch passages and shafts, prioritize collision independently, invalidate only edit dependencies, and retain a conservative collision/admission fallback. This turns streaming from “usually fast enough” into an enforceable gameplay contract.
 
 My first production choice would be Architecture B, while retaining Architecture A’s immutable operator language as the authoring and compilation input. In other words: compose with procedural operators, but execute them into sparse bricks feature-by-feature. It gives up a little theoretical elegance in exchange for much more controllable frame time, edit latency, collision readiness, and debugging.
+
+> Round two, with the code: see `DESIGN-SOL-2026-09-15-CODE.md`. Sol corrected two of the notes above: per-sample lookup is ~5% exclusive (not the main cost), and coarse LOD from fine data is excluded by the clipmap design, not a gap.