Compact single-message option syntax in proto

[tomlinford]

Add support for compact dot notation syntax for field options with single-field nested messages. This makes option declarations more readable and concise.

Examples:

• Before: [(buf.validate.field) = {int32: {gte: 2020}}]

• After: [(buf.validate.field).int32.gte = 2020]

• Before: [(validate.rules) = {string: {uuid: true}}]

• After: [(validate.rules).string.uuid = true]

Implementation:

• Add flattenSingleFieldPath() to recursively detect and flatten single-field nested maps into dot-separated paths
• Update addShortOptions() to apply flattening to field options
• Long options (option declarations) continue to use message literals
• Add comprehensive regression tests

Changes:

• proto_ast/ast.go: Add flattening logic in addShortOptions()
• proto_ast/ast_test.go: Add tests for nested single-field cases
• sroto_ir/example_test.go: Update expected output
• README.md: Update examples to show new compact format
• example/options_example.proto: Regenerate with compact format
• Vendor google/protobuf protos needed for protoc validation

The compact format only applies to field options (inside []), not to long options which properly use message literal syntax:
option (google.api.http) = {get: "/users/{id}"};