ttl: add infix expression parser/type-checker for Go-like operators

[kdy1]

Summary

The lexer defines operator tokens, but parser/sema currently do not implement infix operator parsing and typing with precedence.

Evidence

Contract Evidence:

• docs/project-ttl-language.md:97
• cmds/ttlc/internal/parser/parser.go:354

Current Gap

The lexer defines operator tokens, but parser/sema currently do not implement infix operator parsing and typing with precedence.

Proposed Scope

• Not documented yet.

Acceptance Criteria

• Add AST support for infix/binary expressions.
• Parse +, -, *, / (and precedence/associativity) in expression grammar.
• Add semantic checks for operator operand/result compatibility.
• Produce stable diagnostics for invalid operator combinations.

Test Scenarios

• Precedence test: multiplication binds tighter than addition.
• Parenthesized expressions override default precedence.
• Invalid operand type combinations emit type errors.

Out of Scope

• Full Go operator surface beyond agreed phase scope.
• Constant-folding optimization.