Statically-known existence conditions: drop or simplify the check

Many Emboss schemas have fields whose existence_condition the front
end has already constant-folded to a boolean literal:

  * \`if false:\` fields are never present, so Ok() needs no check
    for them at all. PR 241 still emitted the full has_X() guard +
    Ok() check; both branches are dead.
  * Unconditional fields (and \`if true:\` fields) carry a literal
    \`true\` existence. has_X() is always Known and always true, so
    the standard ok_method_test pattern

      if (!has_x().Known()) return false;
      if (has_x().ValueOrDefault() && !x().Ok()) return false;

    collapses to a bare

      if (!x().Ok()) return false;

A fast pre-pass in _generate_optimized_ok_method_body recognizes
both shapes (existence_condition.type.boolean has a literal value)
and routes them to the short emit paths. \`if false:\` fields are
skipped; the always-true case emits the bare Ok() check.

Wide golden churn: most structs have several unconditional fields
(virtual size fields like IntrinsicSizeInBytes/Max/Min plus any
non-conditional payload), so most Ok() methods shrink by several
lines apiece.

The docstring on _generate_optimized_ok_method_body is rewritten to
describe the four emit paths the function now routes fields through
(skipped / unconditional / switch arm / ok_method_test) — the
original PR 241 docstring referenced only two paths and a duplicate-
case fallthrough that no longer exists.