passes: implement control-flow pass (4 schemes + rotation)

FlowPass rebuilds one method per scheme picked from FlowConfig:
state-spread (dense tableswitch dispatcher), computed-switch
(sparse lookupswitch over a counter local), butterfly (branch
inversion), exception-goto (dispatcher under try with a dead
catch-all), plus opaque goto-splices inside every method and
StackMapTable recomputation afterwards. Now in standard_pipeline
(between constants and noise); --no-flow is an honest off-flag.

Acceptance is real JVM, not shapes: per-scheme verify, rotation
spread, calc-prints-45 execution, paranoid smoke on lorep-1.1.0
(131 methods rebuilt, no new failures over standard).

Two verifier rules learned the hard way are now construction
invariants: no inserted mark shares a bytecode offset with a
foreign one (nop pads), and no frameless gap follows a terminal
(dead blocks are opaque-entered and throw).

Also fixes a latent frame-emitter bug the pass exposed: wide
stack values were written as slot pairs ([Double, Top]) while
javac writes one entry ([Double]); HotSpot rejects the pair.
Stacks now collapse like locals (emit::stack_vars), with a
regression test in classfile/tests/frames.rs.

docs/learnings/priorities.md

@@ -115,6 +115,6 @@ writer).
 ✅ P0 DebugInfo + Resources      → главные утечки сняты
 ✅ P1 Strings (B2) + Constants   → ключ выводится, схемы ротируются
 ✅ P1 backend (ristretto) + frames
-⬜ P2 Flow (C1) — единственное, что ждёт реализации; верификатор готов
+✅ P2 Flow (C1) — готов (09.10.2026): 4 схемы + ротация + мешанина, приёмка настоящим `java`
 ✅ P3 метрики, красные паттерны, CLI
 ```