Completes comptime-cursor tuple indexing (started by the read path in
fee86adf) and unblocks the `race` runtime synthesis. Five enablers:
1. Named-tuple-literal type inference preserves element NAMES. A
`.(a = x, b = y)` passed DIRECTLY as a `$T` argument inferred to a
tuple with `.names = null`, so `field_name(T, i)` reflected "" and a
`make_enum` over those labels collided on the empty name. The typer
now mirrors `lowerTupleLiteral`'s name capture.
2. `inferExprType` resolves a comptime-constant tuple index to the i-th
field's CONCRETE type (the inference sibling of the fee86adf read
path), so `tup[i].field` / methods / comparisons on it resolve.
3. Tuple-element L-VALUES by comptime index — `tup[i] = v`,
`tup[i].f = v`, `@tup[i]` — lower to a typed `structGep` of field i
across all four paths (`lowerAssignment`, the multi-assign store,
`lowerExprAsPtr`, and address-of-index). Previously each emitted an
`index_gep` with a `ptrTo(.unresolved)` element type (a tuple has no
uniform element) that panicked at LLVM emit. An out-of-range comptime
index now diagnoses loudly on every path instead of falling through to
that panic.
4. A user generic `($X..) -> Type` call is recognized as type-shaped
(`isTypeReturningCallNode`), so it can bind a `$E: Type` parameter —
e.g. `make_variant(RaceResult(T), i, …)`. The static
`isTypeShapedAstNode` only knew the type-returning builtins
(field_type/pointee/type_of).
Locked by examples/comptime/0652 (read, fee86adf) and 0653 (store +
address-of + element-pointer field store).
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