Move the structural / non-call arms of Lowering.inferExprType into a new
src/ir/expr_typer.zig (ExprTyper): literals, unary/binary ops, try/catch, if,
block, field access, identifier/type-name, struct/tuple literals,
index/slice/deref, null-coalesce, caller_location, and the no-value statement
shapes. ExprTyper is a *Lowering facade (Principle 5, same as PackResolver) —
expression typing reads live lexical-scope / pack / target-type state and ~14
resolver helpers, so it borrows *Lowering rather than re-threading every field;
the plan's TypeResolver/ProgramIndex/ResolveEnv ideal is the later-phase target
as that state lifts into an explicit context (documented in the module doc).
Lowering.inferExprType is now a 2-arm dispatcher: `.call => inferCallType(c)`
(call result typing stays in Lowering until A3.2), else delegates to
ExprTyper.inferType. The call arm body moved verbatim into the new
Lowering.inferCallType (the by-value `|c|` capture became a `*const ast.Call`
param; the lone `&c` -> `c`).
14 Lowering helper methods consumed by the facade were widened to pub
(orIsFailableChain, orChainSuccessType, errorChannelOf, failableSuccessType,
isObjcClassPointer, lookupObjcPropertyOnPointer,
lookupObjcDefinedStateFieldOnPointer, getElementType, optionalOfFlattened,
getStructFields, isKnownTypeName, comptimeIndexOf, packArgNodeAt, resolveType)
plus Scope.lookup — the same pub-for-facade step PackResolver took. Fields need
no change (Zig fields are always cross-file accessible).
expr_typer.test.zig adds focused unit tests (literal shapes, comparison vs
arithmetic, unary not/negate, deref of non-pointer) for the scope-free
structural arms. Barrel-wired in ir.zig.
Behavior-preserving. Gate: zig build, zig build test (incl. new ExprTyper
tests), bash tests/run_examples.sh -> 356/0. lower.zig ~18774 -> 18598.
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