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.

pipelines/deploy.yaml

@@ -27,7 +27,7 @@ stages:
 
   - name: resolve-stack
     description: Resolve the contract to a Target Stack instance via the contract resolver
-    command: python3 acdl_platform/contract_resolver.py contracts/static-asset.yaml /tmp/acdl-stack.json
+    command: python3 core/contract_resolver.py contracts/static-asset.yaml /tmp/acdl-stack.json
     required: true
 
   - name: terraform-plan
@@ -42,7 +42,7 @@ stages:
 
   - name: confidence
     description: Compute the confidence signal from policy + validation inputs
-    command: python3 acdl_platform/confidence_signal.py /tmp/acdl-deploy-inputs.json dev
+    command: python3 core/confidence_signal.py /tmp/acdl-deploy-inputs.json dev
     required: true
 
   - name: apply