cc: split linearize.rs into 4 focused files

Split linearize.rs (8,600→4,266 lines) into:
- linearize.rs: core, types, expressions, dispatch
- linearize_init.rs (1,094): initializer/declaration linearization
- linearize_stmt.rs (1,904): statement linearization
- linearize_emit.rs (1,399): IR emission helpers

Pure mechanical split — no logic changes. Linearizer fields
changed to pub(crate) so sibling modules can access them.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jgarzik