refactor(ir): extract ErrorFlow (error_flow.zig) for path-sensitive error-flow diagnostics (A5.2 step 2)

Move the diagnostic-only Pass 1e (ERR E1.7 cleanup-absorption + E1.8 value-slot
liveness) out of lower.zig into src/ir/error_flow.zig behind an ErrorFlow
*Lowering facade (Principle 5, like ErrorAnalysis/CoercionResolver). Behavior
preserved exactly — pure relocation.

Moved verbatim (self. -> self.l. for Lowering members; sibling calls stay on the
facade; provenHas is a file-local free fn): checkErrorFlow, analyzeFnBody,
flowWalk, flowStmt, flowIf, flowMatch, flowExpr, applyRefinement,
provenAdd/provenClone/provenIntersect, registerFailableDestructure,
checkCleanupBody/checkCleanupNode/cleanupReject, plus the FlowCtx/ProvenSet types.

- lowerRoot routes the single call site through
  self.errorFlow().checkErrorFlow(decls); no Lowering wrapper kept (only the
  pipeline calls it, no unit-test caller). New errorFlow() accessor.
- The pass takes AST decls + ProgramIndex + diagnostics only — independent of IR
  Builder state (PLAN-ARCH A5.2 success criterion).
- New pub: exprIsFailable (only widening; inferExprType/errorChannelOf already
  pub). lower.zig -389 (->17030); error_flow.zig 407. Barrel-wired in ir.zig.
- No .test.zig: diagnostic-pass altitude (functions return only bool + emit
  diagnostics) — guarded by example anchors 1046-1053 (incl. scaffolding
  1051/1052/1053). Phase A5 complete.

Gate: zig build, zig build test, bash tests/run_examples.sh -> 361/0
(anchors 1046-1053 all ok, no .ir churn).

src/ir/ir.zig

@@ -13,6 +13,7 @@ pub const generics = @import("generics.zig");
 pub const protocols = @import("protocols.zig");
 pub const conversions = @import("conversions.zig");
 pub const error_analysis = @import("error_analysis.zig");
+pub const error_flow = @import("error_flow.zig");
 pub const semantic_diagnostics = @import("semantic_diagnostics.zig");
 
 pub const TypeId = types.TypeId;
@@ -52,6 +53,7 @@ pub const ProtocolResolver = protocols.ProtocolResolver;
 pub const CoercionResolver = conversions.CoercionResolver;
 pub const CoercionPlan = conversions.CoercionResolver.CoercionPlan;
 pub const ErrorAnalysis = error_analysis.ErrorAnalysis;
+pub const ErrorFlow = error_flow.ErrorFlow;
 pub const ErrorFacts = error_analysis.ErrorFacts;
 
 pub const compiler_hooks = @import("compiler_hooks.zig");