Add new specialized `Column` subclass for system columns.

[giant0791]

Core system columns

Identity and container

Notion API	Proposed Column	Rationale
id	object_id	Avoid collision with user id columns
parent.database_id	database_id	Mirrors tableoid semantically

Lifecycle

Notion API	Proposed Column	Notes
in_trash	is_deleted	SQL-idiomatic
archived	is_archived	Keep as-is

Temporal metadata

Notion API	Proposed Column
created_time	created_at
last_edited_time	updated_at

SystemColumn as specialized version of Column:

class Column(...):
    is_system: ClassVar[bool] = False

class SystemColumn(Column):
    is_system: ClassVar[bool] = True

Columns accessors new API

table.c                   # user-defined columns only
table.columns     # alias of table.c
table.sys               # system columns only
table.all                # optional convenience

Storage model:

class Table:
    _user_columns: ColumnCollection
    _system_columns: ColumnCollection

Implication on __len__

len(table) should return the number of user-defined columns only.
Add a new specialized function for returning schema size:

def schema_size(table: Table) -> int:
    return len(table.all)

Name collision and enforcement rules

System columns names are reserved

if name in system_column_names:
    raise SchemaError("Cannot redefine system column")

User cannot shadow system columns

This avoids:

Column("object_id", String)

Which would be catastrophic.