Document the Go test-result cache as a second, independent cache

Go's test-result cache sits below the Docker layer cache that issue #26
addresses. CHECK_EPOCH forces the RUN make test step to re-execute, but
GOCACHE baked into earlier image layers survives into the re-executed
step, so go test can still do no work. Record that these are two
separate defects so a fix for one is not recorded as covering the other.

Also correct the TODO.md Completed Steps date to the landing date, which
matches the REPO_POLICIES.md last_modified front matter.

Closes #44

TODO.md

@@ -21,9 +21,10 @@ fmt-check, and commit.
 
 # Completed Steps
 
-- 2026-08-10: Added `-count=1` to both `go test` invocations in the canonical Go
+- 2026-09-03: Added `-count=1` to both `go test` invocations in the canonical Go
   `make test` example in `REPO_POLICIES.md`, so the target cannot report a
-  cached pass it did not earn.
+  cached pass it did not earn, and documented that Go's test-result cache is a
+  second, independent cache stacked below the Docker layer cache.
 - 2026-08-07: Set the canonical `.golangci.yml` to the org-standard v2-schema
   config already deployed byte-identical across the org's Go repos (settings
   under `linters.settings` so thresholds like lll/funlen/cyclop/dupl actually