Eliminate redundant residual re-read in optimized Ok() switch arms (#272)

Lever A of the Ok() rework: in a conditional-switch arm whose field carries
a residual predicate beyond the discriminant equality (e.g.
`if tag == K && outer == 1`), gate the field's Ok() check on the residual
alone rather than re-deriving the full has_<field>() existence condition.

Inside `case K:` the switch has already decoded and (provably, or via the
emitted Known() guard) established the discriminant, so
`has_<field>() <=> residual`. The prior form re-invoked has_<field>(), which
redundantly re-reads and re-compares the discriminant on every arm; the
residual (a single hoisted, shared subexpression) drops that per-arm work.

Implementation: preserve the residual conjunct list through
_generate_optimized_ok_method_body instead of collapsing it to a bool, thread
(ir, subexpressions) into _emit_switch_block/_render_case_body, and render the
residual at emit time (so demoted groups leak no dead subexpression). Multiple
conjuncts fold into one synthesized boolean AND so the residual renders once.
The gate is required for correctness, not just size: <field>() returns a null
view when absent, so an unguarded <field>().Ok() would wrongly fail when the
residual is false; we also bail when the residual is not Known.

Adds testdata OuterGuardedUnion (100 tag arms each guarded by a shared
`outer == 1` residual) to exercise and measure the change; flat
LargeConditionals (all bare arms) is the unaffected control.

Measured OuterGuardedUnion::Ok() reader symbol @ -Os vs the pre-Lever-A
generator (schema held fixed): MicroBlaze LE/Vitis (authoritative) -18.7%,
MicroBlaze BE -21.5%, ARM Cortex-M4 -17.0%, x86-64 gcc -9.5% / clang -50.7%.
Retired guest instructions (OuterGuardedUnion hot loop): MicroBlaze BE -3.2%,
MicroBlaze LE -1.6%, ARM -1.1%. Flat LargeConditionals codegen is
byte-identical (0 symbol delta on every target); x86 wall-clock unchanged.
Golden diff is confined to the residual-arm schemas (condition.emb.h,
many_conditionals.emb.h); every pre-existing struct is byte-identical. Full
compiler+runtime suite (205 tests) and the #266 condition_test oracle stay green.