Cache substrate aligned with Bazel/REAPI two-cache model

The cache layer now follows the Bazel/REAPI two-cache model: one content-addressed blob store plus one action cache. The experimental kv/ keyed-blob namespace and related *_keyed_* methods have been removed. ActionResult.stdout and stderr are now optional, so writes that did not capture output (or had nothing worth recording) leave those fields unset instead of materialising an empty blob. Tuist cache reads that reference an unavailable blob now correctly count as a cache miss.