feat(prepare): accept comma-separated --target list (image-bake friendly)

[zackees]

Summary

soldr prepare --target all requires a workspace Cargo.toml with [workspace.metadata.soldr].targets populated. Docker-image bake steps don't have one mounted yet — the source volume is mounted at docker run time, not at docker build time.

Extend --target to accept three shapes:

• <triple> — single triple (unchanged)
• <a>,<b>,<c> — explicit comma-separated list (new)
• all — workspace-metadata resolution (unchanged)

Why this matters

The new comma-separated form lets a Dockerfile bake the full cross-toolchain into the image without inventing a placeholder Cargo.toml:

RUN soldr prepare --target \
      x86_64-pc-windows-msvc,aarch64-apple-darwin,x86_64-unknown-linux-musl,...

That's exactly what the upcoming examples/docker-cross-all/ recipe (#924) needs — it has no source volume mounted at image bake time.

Implementation

• Parsing in prepare_cmd::parse_target_arg returns ParsedTargetArg::All or ParsedTargetArg::Explicit(Vec<String>).
• main.rs dispatches on the enum: All → resolve_all_targets(), Explicit → use as-is.
• Whitespace around comma-separated entries is trimmed; leading / trailing / consecutive commas are silently dropped (copy-paste tolerance); an all-empty list errors instead of producing a zero-target run.

Tests

Six new unit tests cover the parser. Existing CLI surface unchanged; the legacy single-triple and all shapes pass through identical to before.

running 6 tests
test prepare_cmd::tests::parse_target_arg_all_is_sentinel ... ok
test prepare_cmd::tests::parse_target_arg_single_triple ... ok
test prepare_cmd::tests::parse_target_arg_drops_empty_entries ... ok
test prepare_cmd::tests::parse_target_arg_comma_separated ... ok
test prepare_cmd::tests::parse_target_arg_trims_whitespace ... ok
test prepare_cmd::tests::parse_target_arg_all_empty_errors ... ok

Refs #914.

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