Verbatim relocation of the 53-method error cluster (error typing,
raise/failable, try/catch/or, inferred-set convergence, trace runtime
hooks) out of the Lowering struct into src/ir/lower/error.zig as free
functions taking *Lowering. Each gets a pub-const alias on Lowering, so
every call site compiles unchanged (decl-alias method resolution).
Pub-flips (callees now referenced cross-file): lowerExpr, coerceToType,
freshBlock, freshBlockWithParams, emitErrorCleanup,
currentBlockHasTerminator, lowerBlock, lowerBlockValue.
Gate: zig build OK; zig build test 426/426; run_examples 541/0; zero
expected/ snapshot churn.
