feat: self-host cond macro, remove Expr::Cond from compiler

[thsfranca]

Summary

• Define cond as a variadic defmacro in stdlib/prelude.vx that expands to nested if expressions
• Remove Expr::Cond, CondClause, parse_cond, expand_cond, and check_cond from the compiler
• cond is no longer a special parser form — it parses as a regular function call and expands via the prelude macro
• Add macro expansion to the typechecker check_source test helper so cond-related tests work through the full pipeline

Net result: -283 lines, +45 lines — the compiler gets simpler while cond keeps working identically.

Test plan

• All 551 tests pass
• cargo clippy clean
• (cond test1 val1 test2 val2 :else default) expands correctly to nested if
• (cond) with no clauses produces nil
• Non-bool cond test still errors at type-check time
• examples/let-and-cond.vx compiles successfully
• Interpreter cond_expression test passes end-to-end