@type default_value() ::
  :none
  | {:literal, nil | boolean() | integer() | float() | String.t()}
  | {:blob, binary()}
  | {:current, :time | :date | :timestamp}
  | {:expr, String.t()}

A column default, classified from the raw SQL text SQLite stores.

• :none - the column declares no default at all.
• {:literal, value} - a parsed literal: nil (explicit DEFAULT NULL), a boolean (DEFAULT TRUE / FALSE — SQLite stores these as INTEGER 1/0), an integer (including hex literals, interpreted as 64-bit two's complement like SQLite does), a float, or a string (with SQLite's '' escaping undone).
• {:blob, binary} - an x'...' hex blob, decoded. May contain arbitrary bytes, including invalid UTF-8 — that is the point of blobs.

• {:current, :time | :date | :timestamp} - the CURRENT_TIME / CURRENT_DATE / CURRENT_TIMESTAMP keywords (matched case-insensitively).

• {:expr, sql} - anything else, verbatim as SQLite stored it: parenthesized expression defaults arrive with their outer parentheses stripped by SQLite (e.g. "datetime('now')", "1+2"). Never constant-folded — SQLite folds at insert time, not xqlite. Integer-shaped values that overflow 64 bits and non-finite floats (9e999) also land here rather than silently changing numeric type.

Note the same literal can store differently depending on column affinity (DEFAULT 42 on a TEXT column inserts the text '42'); the classification reports the default as written, not as any particular column would store it.

Date/time-looking strings (DEFAULT '2024-01-15') stay {:literal, "2024-01-15"} — xqlite does not divine types at the schema layer.

@type t() :: %Xqlite.Schema.ColumnInfo{
  column_id: integer(),
  declared_type: String.t(),
  default_value: default_value(),
  hidden_kind: Xqlite.Schema.Types.column_hidden_kind(),
  name: String.t(),
  nullable: boolean(),
  primary_key_index: non_neg_integer(),
  type_affinity: Xqlite.Schema.Types.type_affinity()
}

Struct definition.

• :column_id - The zero-indexed ID of the column within the table.
• :name - Name of the column.
• :type_affinity - The resolved data type affinity (see t:Types.type_affinity/0).
• :declared_type - The original data type string exactly as declared in the CREATE TABLE statement (e.g., "VARCHAR(50)", "INTEGER", "BOOLEAN").
• :nullable - true if the column allows NULL values, false otherwise (derived from NOT NULL constraint).
• :default_value - The column default, classified (see default_value/0).
• :primary_key_index - If this column is part of the primary key, its 1-based index within the key (e.g., 1 for single PK, 1 or 2 for compound PK). 0 if not part of the primary key.
• :hidden_kind - Indicates if and how a column is hidden/generated (see t:Types.column_hidden_kind/0).