compiler: emit i64.eqz for zero-equality comparisons in WAT codegen

[yevbar]

Summary

Add a peephole optimization to the WAT code generator that emits the i64.eqz instruction when comparing an i64 expression against literal 0 for equality. This saves one instruction (i64.const 0) per zero-comparison in the generated WASM binary.

What changed

WAT code generator (lib/firebird/compiler/wat_gen.ex):

• Added peephole patterns for {:binop, :eq, expr, {:literal, 0}} and {:binop, :eq, {:literal, 0}, expr} that emit i64.eqz instead of i64.const 0; i64.eq
• Applied same optimization to case clause pattern matching against literal 0 (both normal and TCO paths)
• Extracted generate_binop_expr/7 helper for peephole fallback

Before / After

;; Before (3 stack ops):
local.get $x
i64.const 0
i64.eq

;; After (2 stack ops):
local.get $x
i64.eqz

Impact

This pattern appears in:

• Boolean/zero checks: if x == 0
• Case dispatch: case x do 0 -> ... end
• Guard conditions and parity tests (rem(n,2) == 0 after strength reduction becomes band(n,1) == 0)
• Collatz-style algorithms, factorial base cases, etc.

Tests

• 7 new tests in test/compiler/eqz_peephole_test.exs covering WAT output verification, commutative patterns, case expressions, optimized mode, and WASM binary validity
• Updated existing wat_gen_edge_cases_test.exs to expect i64.eqz for pattern-0 cases
• Full test suite passes (9379 tests, only pre-existing xmerl env failure)