Type AST: remove internal string boundaries

[frendsick]

Parent PRD

#140

What to build

Audit remaining Type ↔ str conversions and keep only the ones at true IO boundaries:

• error message formatting
• JSON serialization (LSP, debug dumps)
• source-text consumers (Op.type_annotation, Op.deferred_return_type)

Drop every other internal parse_type / Type.format round-trip that exists only because the migration plumbed strings through intermediate calls.

Acceptance criteria

• No internal Type → str → Type round trips remain (grep parse_type and Type.format call sites)
• Conversion sites limited to: error builders, JSON encode/decode, source-text annotation parsing
• tests/test_examples.sh + tests/test_compiler.sh (< /dev/null) pass
• simplify review pass before PR

Blocked by

• Blocked by Type AST: replace string helpers with Type methods #166

User stories addressed

Migration step 3 final cleanup in parent PRD.