stack-encrypt refactor: extend owned target conversions to collections #1482

#1470/#1404 establishes owned scalar conversion. Collections still need the same owned target model without sacrificing batching or cleanup.

Acceptance:

• Support consuming Vec to Vec through EncryptInto/DecryptFrom and EncryptTarget/DecryptSource, including Vec to Vec without caller context.
• Preserve one logical batch, per-value authentication boundaries, ordering, empty collections and provider chunk limits.
• Propagate explicit context-input capabilities; do not infer optionality from Default.
• Guard owned plaintext through construction failure, partial execution and cancellation; avoid an unnecessary Clone bound.
• Keep direct keyset.encrypt restricted to one opaque value and keep structural encrypt_tree explicit.
• Resolve byte-element/nullable capability constraints with Vitamin C explicitly; do not silently treat Vec as a single opaque ciphertext.
• Add executable public examples, compile-fail capability tests and conformance coverage.