v0.12.1 — chore: remove redundant comments

Removed comments added during the moderate/low audit fix pass that restated what the code already says — section dividers, what-the-function-does docstrings, and variable restatements. Kept non-obvious rationale, safety-critical path explanations, and platform-specific quirks (OKX 51400 idempotent cancel, WebSocket absolute URL requirement, SSRF guard, fill-ring lag on flatten sizing, read-only rate-limit check).