refactor: idiomatic cleanup of EvisionPrecompiled deploy path

[cocoa-xu]

Follow-up to #329. Idiomatic cleanup of the precompiled download → verify → unarchive → deploy flow in Mix.Tasks.Compile.EvisionPrecompiled. Net -74 lines, no functional change to the happy path.

Correctness

verify_checksum/3 returned a 3-tuple {matched?, algo, hash} on success but a 2-tuple {:error, reason} when the file was absent from the checksum map. Both call sites destructured a 3-tuple, so a cached tarball whose basename wasn't listed raised a MatchError instead of surfacing the "run mix evision.fetch" hint. It now returns a consistent :ok | {:error, reason} and owns its diagnostic message; callers case on it.

Idiomatic cleanups

• nif_installed?/1 collapses the identical .so/.dll path + existence check that was duplicated verbatim between prepare/8 and run/1.
• deploy_from_dir!/2 drops redundant if File.exists? guards (File.rm_rf! is already a no-op on missing paths) and uses File.cp_r! in place of a with/else/raise block.
• unarchive!/2 and run/1 replace single-clause with/else with case (per Elixir's anti-patterns guide).
• prepare/8 derives the tarball name as name <> ".tar.gz" instead of a second 8-arg filename/8 call, and the download block reads as a linear sequence.

Scope

Intentionally left the long positional parameter lists (filename/get_download_url/prepare/run) as-is. Modeling them as a build-spec struct would be gold-plating for a build script, so it was skipped rather than abstracted for its own sake.

Verification

• Isolated compile under Elixir 1.20.1 is warning-free.
• The refactored region passes mix format.
• Every caller of the changed functions lives in mix.exs and uses the new interfaces; the sibling mix evision.fetch task does not touch any of them.