refactor(catalog)!: narrow public metadata API

[e54-bot]

Summary

• make Catalog storage, schema, and provenance representation private while retaining stable identity and locale queries
• hide CatalogEntry metadata representation and add stable queries for parameters, defaults, and variadic signatures
• migrate workshop-rs internals and tests without changing parser, validation, localization, or catalog semantics
• keep the semver gate strict for 1.x while allowing approved pre-1.0 API-hardening transitions to advance to the next 0.x compatibility line

Downstream direct-field migrations remain consumer-owned per #255.

Verification

• cargo fmt --all --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo run -p workshop-rs --bin workshop-catalog-gen -- check
• actionlint .github/workflows/ci.yml
• git diff --check
• independent ablation: removing param_default behavior failed the focused catalog test, then the restored implementation passed
• cargo semver-checks check-release -p workshop-rs --release-type major

Fixes #255

[Teakowa]

One blocking issue.

[Teakowa]

LGTM