Drop redundant Known() guard when discriminant is provably Known (#257)

The optimized switch block opens with

    const auto emboss_reserved_switch_discrim = \${discriminant};
    if (!emboss_reserved_switch_discrim.Known()) return false;

The Known() guard is a safety net: if reading the discriminant
failed, Ok() must return false. But fields_in_dependency_order
guarantees the discriminant field is validated earlier in Ok(), and
for a non-conditional, non-virtual field of the current structure
the earlier \`if (has_X().ValueOrDefault() && !X().Ok()) return false;\`
already enforces X().Ok(). Past that point the Maybe<> wrapper around
the field read is provably Known and the extra guard is dead code.

_is_discriminant_provably_known recognizes the safe shape — a
single-segment field_reference to a non-conditional non-virtual field
in the current structure's field list — and removes the guard in
that case. The detection is intentionally narrow; nested paths and
computed discriminants keep the guard.

One line saved per qualifying switch — minor on hosted targets, but
on Thumb-2 and MicroBlaze this removes a load, a compare, and a
conditional return per switch block, plus the constant-pool slot.