Resolves the ordered list of candidate fetch URLs for an attachment: explicit
supported+allowed blossom-v1 locator URLs first, then policy fallback URLs.
Deduplicates while preserving order. A client tries them in order until one
yields bytes matching ciphertextSha256.
Resolves the ordered list of candidate fetch URLs for an attachment: explicit supported+allowed
blossom-v1locator URLs first, then policy fallback URLs. Deduplicates while preserving order. A client tries them in order until one yields bytes matchingciphertextSha256.