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.

contracts/static-asset.yaml

@@ -7,7 +7,7 @@
 #   inputs:     module-specific inputs
 #
 # Validated against schemas/contract.schema.json.
-# Resolved by acdl_platform/contract_resolver.py to a Target Stack instance.
+# Resolved by core/contract_resolver.py to a Target Stack instance.
 
 uses: acdl/pipelines/deploy.yaml@v1.4
 module: static-asset