Extend switch matcher: disjunction, conjunction, single-entry demotion

[AaronWebster]

PR #241's _get_switch_candidate only matched bare discrim == K equalities. Real protocol grammars express the same intent in many forms; this PR rewrites the matcher to recognize them all.

_extract_switch_arms decomposes an existence_condition into a (discriminant, [(case_value, residual), …]) tuple, handling:

• Bare equality (tag == K) — one arm, no residual.
• Disjunction of equalities on a shared discriminant (tag == A || tag == B || tag == C) — one arm per Ki, no residual. Common for tagged unions where several values share a payload. Combined with Sort switch cases by value; coalesce identical-body cases #254's identical-body coalescing, this collapses to a single multi-label switch arm.
• Conjunction with an equality (tag == K && other_predicate) — one arm carrying [other_predicate] as residual. Useful for nested guards like if outer_flag && tag == K.
• Mixed shapes inside a disjunction: (tag == 0 && a) || (tag == 1 && b) || tag == 2 — three arms with respective residuals [a], [b], [].

An arm with a residual emits the has_${field}()-based check as its case body (the residual is folded into the existing accessor) — sound and lets the C++ compiler fold the case-pinned discriminant comparison via inlining.

A demotion pass measures total arm-entries per group and falls back to ok_method_test when the count is below 2 — without this, a lone field like if outer && tag == K: xc would get wrapped in a one-case switch whose overhead (temporary, Known() guard, scope braces) exceeds the dedupe. This also fixes a latent bug from #253: the scoped discriminant render mutated subexpressions during the grouping pass, so groups that later got demoted would have left dead const auto = …; definitions in the emitted Ok(). The scoped render now happens at emit time, only for surviving groups.

The benchmark schema gains a DisjunctionConditionals struct exercising three || chains (2, 3, 3 labels) and the benchmark TU gains a runtime test for it. Golden churn: many_conditionals.emb.h gains the new struct's output; condition.emb.h and virtual_field.emb.h shrink because several BasicConditional-style structs had a lone xc field that PR #241 was wrapping in a one-arm switch — demotion brings them back to the cheaper has_xc() check.

Size impact (cumulative vs. master)

Target	Metric	Master	PR	Delta
ARM Thumb-2	TU .text	18962	18208	−754 (−4.0%)
ARM Thumb-2	LargeConditionals::Ok()	5382	4746	−636 (−11.8%)
ARM Thumb-2	DisjunctionConditionals::Ok()	358	270	−88 (−24.6%)
MicroBlaze	TU .text	43640	42612	−1028 (−2.4%)
MicroBlaze	LargeConditionals::Ok()	14824	14104	−720 (−4.9%)
MicroBlaze	DisjunctionConditionals::Ok()	640	552	−88 (−13.8%)
Host x86-64	TU .text	29166	28304	−862 (−3.0%)
Host x86-64	LargeConditionals::Ok()	3948	3065	−883 (−22.4%)
Host x86-64	DisjunctionConditionals::Ok()	431	375	−56 (−13.0%)

Incremental vs. #254: DisjunctionConditionals::Ok() drops 24.6% on Thumb-2, 13.8% on MicroBlaze, 13.0% on x86-64 — the matcher extensions account for nearly all of those.

Stacked on #254.