Update docs to reflect module directory restructuring
ARCHITECTURE.md crate structure tree now matches the actual layout
after splitting crypto.rs, records.rs, and xrpc.rs into module
directories. Also adds files that were missing from the tree
(download_grant.rs, download_keyring.rs, sharing/list.rs, etc.).
CONTRIBUTING.md testing section updated to mention the extracted
test file pattern used by larger modules.
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