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/getting-started.md

@@ -75,6 +75,21 @@ cmake --build build -j$(nproc)
 > **Note:** `-DWITH_CGAL=ON` implies `-DWITH_VIEWER=ON`. Do not use this in headless
 > environments — it will fail with `Failed to find wayland-scanner`.
 
+### Mode 4 — Low-memory build (RAM-constrained CI / Raspberry Pi ≤ 4 GB)
+
+For machines where the CGAL build OOMs (peak ~700 MB per compilation unit at `-O3`):
+
+```bash
+cmake -S code -B build -DWITH_CGAL_TESTS=ON \
+      -DCONFORMALLAB_LOW_MEMORY_BUILD=ON
+cmake --build build --target conformallab_cgal_tests -j1
+```
+
+`LOW_MEMORY_BUILD` applies four measures: `-O0` (no debug info), PCH off,
+unity batch size 1, `--no-keep-memory` linker flag.  Drops cc1plus peak from
+~700 MB to ~150-200 MB per TU.  Tests run ~15× slower at `-O0` but all pass.
+**Always use `-j1`** — parallel compilation would defeat the memory savings.
+
 ---
 
 ## Running a single test