refactor(compile): use ValueEnum for the --type argument

[studiokvc]

Description

Closes #345.

CompileCommand::script_type was a String limited by value_parser, so clap already rejected anything else and the _ => arm returning "Invalid script type" was unreachable. It is now a ScriptType enum deriving clap::ValueEnum (Sh, Wsh, ShWsh, Tr), following DatabaseType in src/persister.rs, and the match is exhaustive.

Notes to the reviewers

The CLI surface is unchanged: the accepted values are still sh, wsh, sh-wsh, tr (default wsh, TYPE env var kept), an invalid value is still rejected by clap with the same list of possible values, and bdk-cli compile --help is byte-identical before and after. I left the variants without doc comments on purpose, since clap would otherwise switch the help to a per-value list.

Tests:

• two parser unit tests: each value maps to its variant, the default is wsh, and pkh is rejected;
• cargo test --features compiler --bins (17 passed) and the compile integration tests (cargo test --features compiler --test cli compile, 3 passed);
• the default build (without compiler) still compiles; cargo fmt and cargo clippy --features compiler --all-targets are clean.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[vadim-anfv]

This duplicates #346, which addresses #345 with the same approach and was opened earlier.

If you'd like to help, a review on #346 would be welcome.

Closing this one as a duplicate.