Shared toolkit and umbrella type for PB's structured errors.

PB does not raise a single catch-all error struct. Instead each failure category is its own defexception, with typed fields rather than a free-form details map:

• PB.SchemaError — schema-resolution / introspection failures (unknown_message, unknown_enum, unknown_service, unknown_extension, invalid_schema).
• PB.ValueError — input value/shape failures (invalid_value, value_out_of_range, unknown_field, invalid_oneof, enum-value problems, missing_required, message_name_mismatch, strict-projection failures, absent, …).
• PB.OptionError — option failures (unknown_options, invalid_option, invalid_options).
• PB.DecodeError — structurally invalid serialized input (malformed wire bytes, malformed_json, duplicate_field).
• PB.AdapterError — an adapter to_proto/from_proto callback failed.

Non-bang APIs return {:error, error} where error is one of the structs above (t/0); bang APIs raise the same struct. To handle any PB error uniformly, rescue the list of modules or match with is_exception/1.

Each struct carries :operation (the entry point that produced it) and :message_name for context; the value/decode/adapter/schema structs also carry a :path locating the offending field. This module holds the shared pieces: the t/0 union, the operation/path_element types, polymorphic prepend_path/2, and the formatting helpers each struct's message/1 uses.