Fix type inference bugs and deduplicate canonical_type

[devin-ai-integration]

Summary

Fixes several bugs found during code review of recent polyglot type-inference commits:

1. Deduplicate canonical_type (3× copy-paste → 1 method)

Moves the logic to Typ::canonical() on core_ir::Typ. The three call sites (core_ir_verifier, core_typecheck, native_emit/lower) now delegate to it.

2. Fix verifier false positives on Any return types

The verifier used raw equality (canonical_type(&expr_typ) != canonical_type(ret)) which would flag return 42 as a type mismatch in a function declared -> Any. Added Typ::compatible_with() which mirrors core_typecheck's existing type_compatible:

// Before (verifier):
canonical_type(&expr_typ) != canonical_type(ret)  // false positive when ret = Any

// After:
!ret.compatible_with(&expr_typ)  // Any matches everything

3. Fix js_return_type / ruby_return_type: scan nested returns

Both only iterated top-level statements, missing returns inside If/Loop/Match blocks. Extracted shared find_return_expr that recurses into branches.

4. Fix js_return_type Binary op → always Int

Expr::Binary { .. } => Typ::Int was wrong for comparisons (==, <) and logical ops (&&, ||) which should be Bool. Now dispatches by operator.

5. Fix go_return_type greedy DFS

Replaced named_descendant(func, "type_identifier") (searches entire subtree including body) with a direct-child scan that stops at the block node — same pattern as v_return_type.

6. Fix Zig empty body check

node_txt(src, body).trim() == "{}" failed on { } or {\n}. Now strips outer braces and checks if inner content is whitespace-only.

Link to Devin session: https://app.devin.ai/sessions/5d1e59069bb447cd8597fb4a8c6c09ea
Requested by: @undivisible

[devin-ai-integration]

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