Initial K3 snapshot: 0.5B KDA/MLA/MoE train path

Standalone tree split from LLMRL/projects/kda. Includes Triton dt_bias
backward fix, train_k3 --preset 0.5b, SFT, Docker runtime, and tests.

.dockerignore

@@ -0,0 +1,15 @@
+# Build context is projects/kda/. Keep the image a runtime, not a data dump.
+.venv/
+**/__pycache__/
+**/*.pyc
+**/*.pyo
+**/.pytest_cache/
+**/.ruff_cache/
+ckpts/
+*.pt
+notes/
+.git/
+.gitignore
+uv.lock
+pyrightconfig.json
+inspect_tensors.py