Docs overhaul: beginner-first README, three-engine consistency, real->f64

[NiceAndPeter]

Summary

• README rewritten beginner-first (224 → 99 lines): "what is bound for" bullets up top, prominent tutorial link, grouped docs index; single-header + regeneration content moved to new docs/single-header.md, C++20 build mode to docs/freestanding.md
• Docs pass over all 13 hand-written docs: three-engine (dbl/flt/cordic) story fixed in math.md/determinism.md/resources.md/tutorial.md/internals.md; freestanding.md + roadmap.md now reflect shipped BND_MATH_NO_FP; deprecated real spelling migrated to f64/f32; broken links/anchors fixed; diagnostics.md bit table completed against policy_flag.hpp
• Generated docs (accuracy.md, performance.md) untouched

Verification

• All relative links and cross-doc anchors resolve
• README/tutorial snippets compile with g++ -std=c++23 -fsyntax-only -I include
• grep clean: no stale "two engines" claims, no primary-real spellings outside deprecation asides

🤖 Generated with Claude Code