docs: fix stale version, CGAL header, add LOW_MEMORY_BUILD docs (U4+U5+U7+U8)

U4 (README.md:17) — status line: v0.9.0 → v0.10.0, test count 0 → 277

U5 (Discrete_conformal_map.h:6-16) — \file Doxygen block rewritten:
  was: 'provides a single function … Spherical/hyperbolic scheduled for Phase 8b.2'
  now: lists all three discrete_conformal_map_* functions already present,
       plus pointers to the circle-packing companion headers

U7 (getting-started.md) — new 'Mode 4 — Low-memory build' section added
  after Mode 3; shows CONFORMALLAB_LOW_MEMORY_BUILD=ON with -j1 and explains
  the -O0 / no PCH / batch-1 tradeoffs for Raspberry Pi / ≤ 4 GB runners

U8 (README.md compile-time modes) — LOW_MEMORY_BUILD entry added to the
  compile-time workflow code block with a one-line explanation and the
  mandatory -j1 note

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

doc/reviewer/usability-audit-2026-05-31.md

@@ -539,11 +539,11 @@ exposed via the CGAL public API, but a CLI user cannot reach them.
 | U1 | README + 4 examples | Usability | 🔴 Critical | ✅ Fixed 2026-05-31 |
 | U2 | examples/ (missing) | Usability | 🔴 Critical | ✅ Fixed 2026-05-31 |
 | U3 | `contracts.md:16` | Doc error | 🟡 Medium | ✅ Fixed 2026-05-31 |
-| U4 | `README.md:17` | Stale | 🟡 Medium | 🟡 Open |
-| U5 | `Discrete_conformal_map.h:14` | Stale | 🟡 Medium | 🟡 Open |
+| U4 | `README.md:17` | Stale | 🟡 Medium | ✅ Fixed 2026-05-31 |
+| U5 | `Discrete_conformal_map.h:14` | Stale | 🟡 Medium | ✅ Fixed 2026-05-31 |
 | U6 | README + 2 examples | Stale | 🟡 Medium | 🟡 Open |
-| U7 | `getting-started.md` | Missing | 🟠 Minor | 🟡 Open |
-| U8 | `README.md` | Missing | 🟠 Minor | 🟡 Open |
+| U7 | `getting-started.md` | Missing | 🟠 Minor | ✅ Fixed 2026-05-31 |
+| U8 | `README.md` | Missing | 🟠 Minor | ✅ Fixed 2026-05-31 |
 | U9 | `layout.hpp` + example | Missing | 🟠 Minor | 🟡 Open |
 | U10 | `getting-started.md` | Missing | 🟠 Minor | 🟡 Open |
 | U11 | `conformallab_cli.cpp` | Roadmap | 🟠 Minor | 🟡 Open |