descriptor: reject empty enums

[fallintoplace]

What changed

• Rejects empty enum declarations in DescriptorPool with PoolError::EmptyEnum.
• Keeps failed pool additions transactional.
• Keeps one-value closed enums with nonzero numbers valid.

Testing

• cargo test -p buffa-descriptor --all-features --all-targets
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all --check

[github-actions]

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[iainmcgin]

[claude code]

Matches protoc ("Enums must contain at least one value.", checked before the first-value rule, which protoc guards with value_count() > 0) and protobuf-go, keeps a one-value closed enum with a nonzero number valid, and is transactional and nested-covered. CI is green. Two fixups:

• open_enum_with_no_values_is_not_rejected_by_the_first_value_rule (pool_e2e.rs:436) still passes but is now vacuous — an empty enum can no longer reach the rule it guards, and its comment ("protoc rejects an empty enum for a different reason") is stale. Either fold it into the new test or change it to assert Err(PoolError::EmptyEnum { .. }), keeping the comment about why the first-value rule must not be the one that fires.
• Add (#421) to the fragment.

And as on the sibling PRs: please drop the edit to the # Errors paragraphs on DescriptorPool::new / decode; we'll rewrite that sentence once after the series lands rather than merge eight competing versions of it. When #418 is reworked it inserts at the same line in link_enum — EmptyEnum should stay first, which is protoc's order.

[iainmcgin]

[claude code] No push since the review, so I've applied the two items myself so this can ride 0.10.0: c036353 merges main, turns the now-vacuous open_enum_with_no_values... test into an assertion that EmptyEnum (not the first-value rule) is what fires, adds (#421) to the fragment, and keeps main's wording for the DescriptorPool::new/decode error summaries (one rewrite after the series). CI approved and running.