feat(api): add catalog mode option helpers

[swernerx]

Summary

• Refs api: options surface — redundant semantics × plural_encoding pair and copy-pasted option structs #54.
• Adds CatalogMode in ferrocat-po for the three valid high-level catalog combinations: ICU PO, ICU NDJSON, and Gettext PO.
• Adds mode() and with_mode(...) helpers to UpdateCatalogOptions, UpdateCatalogFileOptions, CombineCatalogOptions, and ParseCatalogOptions so callers can keep storage format, semantics, and plural encoding synchronized.
• Updates API docs to prefer CatalogMode for normal construction while keeping the existing explicit fields available.

Verification

• cargo fmt --all --check
• cargo check --workspace --all-targets --all-features --locked
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-features --locked
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --all-features --no-deps --locked
• cargo +1.93.0 check --workspace --all-targets --all-features --locked
• cargo package -p ferrocat-icu -p ferrocat-po -p ferrocat --locked
• pnpm build from docs/
• git diff --check

Notes

• This is a first API-design step, not the full option-struct refactor.
• It intentionally does not close api: options surface — redundant semantics × plural_encoding pair and copy-pasted option structs #54; remaining work is removing or composing duplicated option fields once the safe construction path has landed.
• CatalogMode is exported from ferrocat-po in this PR. The umbrella ferrocat reexport is left out so the existing package verification continues to work against the currently published lower-level crate during same-version release checks.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.06%. Comparing base (9173e0e) to head (c90bc55).

Files with missing lines	Patch %	Lines
crates/ferrocat-po/src/api/types.rs	87.50%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
- Coverage   96.14%   96.06%   -0.09%     
==========================================
  Files          25       25              
  Lines        9904    10000      +96     
==========================================
+ Hits         9522     9606      +84     
- Misses        382      394      +12     

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