tech-debt(fula-mcp): unify read.rs decrypt cores to call the shared fula_crypto::sharing cores (drift surface) #88

Follow-up from the A6b recipient-bindings review (PR #87, GLM finding F3).

Today there are TWO byte-identical copies of the owner-file (encType:fula) decrypt logic:

• native private crates/fula-mcp/src/read.rs: decrypt_single_block / assemble_chunked
• shared public crates/fula-crypto/src/sharing.rs: decrypt_accepted_single_block / assemble_accepted_chunked (called by the fula-js wasm recipient bindings the hosted Worker uses)

They are kept in lockstep by the #[cfg(test)] no-drift cross-check native_and_shared_owner_file_cores_are_byte_identical in read.rs, which (as of #87) covers all three version arms (Some(>=4) / Some(<4) / None) plus the chunked path.

This is correct today but is a latent footgun. Cleanup: refactor read.rs to delegate to the shared sharing:: cores and delete the private copies, eliminating the drift surface entirely.

Deferred from #87 deliberately: the production read path was left untouched to minimize risk to the already-merged local MCP, and the no-drift gate makes deferral safe. Non-blocking / low priority.