fix: flip equality comparisons in place instead of calling a generic helper

[fchimpan]

Go permits x == y between operands of different static types when one is
assignable to the other (e.g. any == error, io.Writer == *os.File, or two
interface types where one embeds the other). The generated generic helper
_mutest_eq_N[T comparable](a, b T) cannot unify such operands into a single
type parameter, so type inference failed and the whole package aborted at
the build stage with zero results (issue #39).

Since the equality mutation is exactly a negation, rewrite the site as

(x == y) != _mutest_on(N)

where _mutest_on reports whether mutant N is active. The original
comparison stays in place, evaluated once, so its typing and semantics
cannot drift:

• mixed static types need no inference (fixes Instrumentation build fails for legal mixed-type equality comparisons (e.g. any == error) #39)
• comparison results stay untyped bool, so defined-bool-type contexts
  (type Flag bool; return a == b) now build too, where the old helper's
  typed bool aborted the package the same way
• recover() in an operand keeps working: unlike the inline closure
  previously used for nil comparisons, the operand is not moved into a
  nested function literal, where recover() no longer stops a panic
• operand text is emitted once, so nested equality grows linearly

This replaces both the _mutest_eq_N helper and the nil-comparison inline
closure. Ordered comparisons keep the generic cmp.Ordered helper: their
mutations are not negations. instrumentFile now also fails loudly if a
mutation point matches no AST node instead of silently reporting a no-op
mutant as SURVIVED.

Fixes #39