Python pickle metadata

`pickle_metadata`

Read-only opcode scan — no unpickling, no imports, no code execution. There is no columns[] array; see column statistics.

Field	Meaning
`byte_count`	File size
`protocol`	First `\x80` + protocol byte when present
`encoding`	`binary` or `text` (protocol 0/1 ASCII)
`protocols_seen`	Every `PROTO` opcode observed
`frame_count`, `frame_bytes_total`	Protocol 4+ `FRAME` opcodes
`referenced_globals`	Deduplicated `module.name` from `GLOBAL`, `INST`, `STACK_GLOBAL`
`builtin_types`	Subset under `builtins.`, `__builtin__.`, `collections.*`
`content_hint`	`tabular`, `ml_model`, `numeric_array`, or `builtin_containers`
`scan_truncated`, `scan_error`	Scan stopped early