ci(py-315): document why cp315 requires no wheels yet

The empty cp315 required-wheel list is coupled to the cp315 allow_failure
rule in .gitlab/package.yml: both must flip together, and #17816 owns that
flip.

Author: vlad-scherbich

.gitlab/package.yml

@@ -106,6 +106,8 @@ variables:
         IMAGE_TAG: *AARCH64_IMAGES
   # TODO(py-315): drop the cp315 allow_failure rule once #17849 ports the bytecode
   # wrapping path to sys.monitoring (currently raises NotImplementedError on 3.15).
+  # The cp315 wheels this matrix produces are kept for CI signal only and are deleted
+  # before every publication step by .gitlab/scripts/prune-unsupported-wheels.sh.
   rules:
     - if: $PYTHON_TAG == "cp315-cp315"
       allow_failure: true

.gitlab/validate-ddtrace-package.py

@@ -51,6 +51,10 @@
 # lose into a hard failure of "ddtrace package" -- which has no allow_failure and
 # gates the release stage. 4.14.0 shipped cp315 musllinux but no cp315 manylinux,
 # and would have been blocked by such a requirement.
+# TODO(py-315): keep this in lockstep with the cp315 allow_failure rule in
+# package.yml. Populating it while that rule stands re-creates the hard failure
+# above; dropping the rule without populating it leaves cp315 unvalidated. #17816
+# owns the flip and must do both at once.
 REQUIRED_PLATFORMS: dict[str, list[str]] = {"cp315": []}
 OPTIONAL_WHEELS = {("cp315", p) for p in BASE_PLATFORMS if "linux" in p}