descriptor: reject duplicate reserved names

[fallintoplace]

What changed

DescriptorPool now rejects duplicate reserved names.

• Checks duplicate reserved_name entries on messages
• Checks duplicate reserved_name entries on enums
• Keeps the same reserved name valid in different owners
• Failed additions stay transactional

Why

A hand-built descriptor set could repeat a reserved name and the pool would silently deduplicate it.

Tests

• cargo test -p buffa-descriptor --features reflect --test pool_e2e
• cargo test -p buffa-descriptor --all-features
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check

[github-actions]

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