refactor(P21): rename acdl_platform/ -> core/ (REQ-53)

---ci---
project: acdl
phase: 21
milestone: v1.6
status: execute
---/ci---

Rename the acdl_platform/ package to core/ across the directory, all
imports in tests/scripts/pipelines/workflows, and doc references. The
package is imported as core.confidence_signal / core.contract_resolver /
core.outbox_writer. The deploy workflow's platform-repo checkout dir is
renamed acdl-platform/ -> platform/ (workspace path, not the python
package). Both .gitea + .github workflows stay byte-identical.

Note: the original target name 'platform/' shadows Python's stdlib
platform module (pytest's import uuid -> platform.system() fails when
the repo root is on sys.path, which every test does). 'core/' avoids
the clash while honoring the intent (drop the verbose acdl_platform).

Tests: 154 pass. run_ci.sh green.

.github/workflows/ci.yml

@@ -35,9 +35,9 @@ jobs:
       - name: Compile all Python files
         run: |
           python3 -m py_compile \
-            acdl_platform/confidence_signal.py \
-            acdl_platform/outbox_writer.py \
-            acdl_platform/contract_resolver.py \
+            core/confidence_signal.py \
+            core/outbox_writer.py \
+            core/contract_resolver.py \
             adapters/terraform/adapter.py \
             adapters/terraform/policy/checkov_adapter.py \
             scripts/push_consumer_image.py