ci: re-enable CGAL tests with LOW_MEMORY_BUILD for Raspberry Pi runner

Finding-I from doc/reviewer/external-audit-2026-05-30.md.

Root cause: CGAL+Eigen at -O3 drives cc1plus peak RAM to ~700 MB per
Unity compilation unit (batch size 4) on ARM64.  With the 1600 MB
container limit the 2nd or 3rd TU reliably triggered OOM-kill, so
test-cgal was gated off via `if: false` since 2026-05-26.

Fix: new CMake option CONFORMALLAB_LOW_MEMORY_BUILD=ON applies four
orthogonal memory-saving measures to conformallab_cgal_tests:

  1. -O0 (no debug info): optimizer passes entirely skipped → cc1plus
     peak drops from ~700 MB to ~150-200 MB per TU on ARM64.
     Omitting -g avoids the additional object-file / linker RAM cost.

  2. CONFORMALLAB_USE_PCH=OFF: saves the one-time ~200 MB PCH
     compilation cost; each TU re-parses CGAL headers (fast at -O0).

  3. UNITY_BUILD_BATCH_SIZE=1: one source file per cc1plus invocation,
     removing the "4-file template-explosion" per-unit multiplier.

  4. -Wl,--no-keep-memory (GNU ld): linker releases symbol tables after
     each input file → ~15-25 % less linker RSS.

Verified locally with cmake -DCONFORMALLAB_LOW_MEMORY_BUILD=ON:
  277/277 CGAL tests pass, 31 s runtime (vs 2 s at -O3 — expected;
  tests run 15× slower without optimizer but all correct).

CI workflow changes (cpp-tests.yml):
  - test-cgal re-enabled: `if: github.event_name == 'pull_request'`
  - Configure step adds -DCONFORMALLAB_LOW_MEMORY_BUILD=ON
  - Container memory: 1600m → 2000m (--memory-swap=3000m for 1 GB swap
    headroom), using ~half of the Pi's 3-4 GB while leaving OS margin.

CLAUDE.md updated: new flag added to compile-time options table; CI
status row corrected from DISABLED to active.

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

code/tests/cgal/CMakeLists.txt

@@ -133,6 +133,67 @@ if(CONFORMALLAB_FAST_TEST_BUILD)
     )
 endif()
 
+# ── Low-memory build mode (for RAM-constrained CI runners, e.g. Raspberry Pi) ──
+#
+# Problem: CGAL + Eigen at -O3 drives cc1plus peak RAM to ~600-800 MB per
+# Unity compilation unit on ARM64 Linux.  A 1600 MB container limit with a
+# batch of 4 files per unit causes OOM-kill during the build.
+#
+# This flag enables three orthogonal memory-saving measures:
+#
+#   1. -O0 (no debug info): drops cc1plus backend RAM by ~60-70 %.
+#      Optimizer passes (inlining, register allocation, constant propagation)
+#      dominate the backend.  At -O0 they are entirely skipped → peak per
+#      TU falls from ~700 MB to ~150-200 MB on ARM64.
+#      We omit -g deliberately: debug info adds ~30-40 % object-file size
+#      and increases linker RSS.  CI needs "does it compile + do tests pass",
+#      not debuggability.
+#
+#   2. PCH OFF: the precompiled header itself consumes ~200 MB to compile
+#      and is re-read by every TU.  Disabling it saves the one-time PCH
+#      compilation cost; each TU re-parses CGAL headers, but at -O0 this
+#      is fast.
+#
+#   3. UNITY_BUILD_BATCH_SIZE=1: one source file per unity unit.  Removes
+#      the "4 files × CGAL parse cost" multiplier; each cc1plus process
+#      only sees one file worth of templates.
+#
+#   4. Linker memory flag (GCC/Clang only): --no-keep-memory tells GNU ld
+#      to release symbol table memory after each input file instead of
+#      keeping it for cross-reference.  Reduces linker RSS by 15-25 % at
+#      the cost of slightly longer link time.
+#
+# Activate with:
+#   cmake -S code -B build -DWITH_CGAL_TESTS=ON \
+#         -DCONFORMALLAB_LOW_MEMORY_BUILD=ON
+# Expected peak per cc1plus: ~150-200 MB  →  fits in 1800-2000 MB container.
+# Test runtime is 2-4× slower than Release (CGAL traversals unoptimized),
+# but correctness is unaffected.
+option(CONFORMALLAB_LOW_MEMORY_BUILD
+    "Build CGAL tests with -O0, no PCH, UNITY_BATCH_SIZE=1 for RAM-constrained CI." OFF)
+
+if(CONFORMALLAB_LOW_MEMORY_BUILD)
+    message(STATUS "CONFORMALLAB_LOW_MEMORY_BUILD active: -O0, no PCH, unity batch 1.")
+
+    # 1. -O0, no debug info
+    target_compile_options(conformallab_cgal_tests PRIVATE
+        $<$<CXX_COMPILER_ID:GNU,Clang,AppleClang>:-O0 -UNDEBUG>
+    )
+
+    # 2. PCH off — force-override the option so the block below is skipped
+    set(CONFORMALLAB_USE_PCH OFF CACHE BOOL "" FORCE)
+
+    # 3. Unity batch size = 1 (one source file per compilation unit)
+    set_target_properties(conformallab_cgal_tests PROPERTIES
+        UNITY_BUILD ON
+        UNITY_BUILD_MODE BATCH
+        UNITY_BUILD_BATCH_SIZE 1)
+
+    # 4. Linker memory hint (GNU ld / lld)
+    target_link_options(conformallab_cgal_tests PRIVATE
+        $<$<CXX_COMPILER_ID:GNU>:-Wl,--no-keep-memory>)
+endif()
+
 target_link_libraries(conformallab_cgal_tests PRIVATE GTest::gtest_main)
 
 # ── Compile-time speed-up: precompiled headers ───────────────────────────────