ci: add memory limits to all containers — Pi protection

Every container now has an explicit --memory limit so the OOM killer
has clean boundaries to work with instead of letting any single job
consume all available RAM:

  test-fast:      800m  (build Eigen+GTest, run 26 tests)
  test-cgal:     2000m  (LOW_MEMORY_BUILD, was already set)
  quality-gates:  600m  (apt install + 4 script checks)
  doc-build:      800m  (Doxygen HTML generation)
  markdown-links: 400m  (pure Python, very lightweight)

memory-swap = 1.5× memory in each case (150% total) so the OOM
killer fires cleanly without exhausting the SD card.

⚠  This is workflow-side protection only.  The runner-side fix
   (capacity: 1 in act-runner config) prevents parallel containers
   entirely and is the more reliable protection — see CLAUDE.md.

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

.gitea/workflows/cpp-tests.yml

@@ -33,6 +33,7 @@ jobs:
     runs-on: eulernest
     container:
       image: git.eulernest.eu/conformallab/ci-cpp:latest
+      options: "--memory=800m --memory-swap=1200m"
 
     steps:
       - uses: actions/checkout@v4
@@ -137,6 +138,7 @@ jobs:
     runs-on: eulernest
     container:
       image: git.eulernest.eu/conformallab/ci-cpp:latest
+      options: "--memory=600m --memory-swap=900m"
 
     steps:
       - uses: actions/checkout@v4

.gitea/workflows/doc-build.yaml

@@ -31,6 +31,7 @@ jobs:
     runs-on: eulernest
     container:
       image: git.eulernest.eu/conformallab/ci-cpp:latest
+      options: "--memory=800m --memory-swap=1200m"
     continue-on-error: true   # never block the merge
     steps:
       - uses: actions/checkout@v4

.gitea/workflows/markdown-links.yml

@@ -31,6 +31,7 @@ jobs:
     runs-on: eulernest
     container:
       image: git.eulernest.eu/conformallab/ci-cpp:latest
+      options: "--memory=400m --memory-swap=600m"
 
     steps:
       - uses: actions/checkout@v4