fix: resolve oneof enum name collisions with nested types

[th0114nd]

Summary

Fixes #31.

When a protobuf message has a nested type (message or enum) whose PascalCase name collides with a oneof enum's PascalCase name, the codegen now suffixes the oneof enum with Oneof instead of returning a hard NestedTypeOneofConflict error.

• oneof region_codes + message RegionCodes → generates RegionCodesOneof enum
• oneof my_field + message MyField → generates MyFieldOneof enum
• Non-colliding names are unchanged
• If the suffixed name also collides with a nested type (pathological case), an OneofSuffixConflict error is returned

The suffix is applied consistently across all code paths: struct field types, enum definitions, view enums (MyFieldOneofView), binary encode/decode, text format, and JSON serde.

Changes

• oneof.rs: Added reserved_names_for_msg() helper and modified oneof_enum_ident() to accept a reserved name set and suffix with Oneof on collision
• message.rs: Builds reserved set, threads through oneof_enum_ident call sites
• impl_message.rs, impl_text.rs, view.rs: Thread msg parameter to compute reserved names at each oneof_enum_ident call site
• lib.rs: Removed check_nested_type_oneof_conflicts() upfront validation and NestedTypeOneofConflict error variant; added OneofSuffixConflict for pathological double-collision case
• tests/naming.rs: Converted error-asserting test to success-asserting with Oneof suffix check; added tests for nested enum collision, view suffix propagation, and double-collision error

Test plan

• Existing test_nested_type_oneof_conflict_resolved_with_suffix — nested message + oneof collision generates MyFieldOneof
• New test_nested_enum_oneof_conflict_resolved_with_suffix — nested enum + oneof collision (gh#31 example) generates RegionCodesOneof
• New test_nested_type_oneof_conflict_view_uses_suffix — view enum uses MyFieldOneofView
• New test_oneof_suffix_double_collision_errors — pathological case errors cleanly
• Existing non-collision tests still pass
• Full workspace cargo test passes (233 tests)
• cargo clippy --workspace -- -D warnings clean
• No WKT regeneration needed (no collisions in well-known types)

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[th0114nd]

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[th0114nd]

Added a follow-up commit addressing a code review finding: sequential oneof ident allocation to prevent sibling oneofs from claiming the same suffixed name.

Example: nested message MyField, oneof my_field → MyFieldOneof, second oneof my_field_oneof → would also produce MyFieldOneof. The fix introduces resolve_oneof_idents() which processes oneofs in declaration order, growing the reserved set after each allocation. All call sites now look up pre-computed idents from a HashMap<usize, Ident> instead of independently calling oneof_enum_ident.