feat: fold field_count/size_of/align_of as comptime constants

The int-returning type-query builtins now fold to a compile-time constant in
const-required positions (`inline for` bound, array dimension), like a plain
`K :: 3` const — previously they evaluated only as runtime values, so
`[field_count(S)]T` and `inline for 0..field_count(S)` were rejected as "not a
compile-time integer". This is what lets a `($T) -> Type` builder loop
`inline for 0..field_count(T)` to assemble a member list from a type's fields
(the `race` result synthesis).

The shared comptime-int folder `evalConstIntExpr` (program_index.zig) gained a
`.call => ctx.evalConstCallInt(node)` arm. The body-lowering ctx (`Lowering`)
implements it — resolve the type arg via `resolveTypeArg`, return
`memberCount orelse 0` / `typeSizeBytes` / `typeAlignBytes`, matching the runtime
value path in lower/call.zig exactly. `SourceConstCtx` delegates to its wrapped
Lowering; the stateless ctxs (`ModuleConstCtx`, `StatelessInner`, test `DimCtx`)
stub null (they cannot resolve a type-expr arg). A non-type-query call / wrong
arg count / unresolved type arg folds to null (not a comptime integer).

Adversarially reviewed (SHIP): the fold matches the value path across every type
kind, ctx coverage is complete, recursion is AST-depth bounded, no speculative
spurious diagnostics, `orelse 0` is field_count's definitional value for
non-aggregates (not a silent default). Locked by
examples/comptime/0648-comptime-typequery-const-fold.sx. Suite green (820/0).

examples/comptime/expected/0648-comptime-typequery-const-fold.exit

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+0

examples/comptime/expected/0648-comptime-typequery-const-fold.stderr

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+

examples/comptime/expected/0648-comptime-typequery-const-fold.stdout

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+field_count(S) loop sum = 3
+array[field_count(S)] len = 3  xs[2] = 20
+field_count(E) = 4
+size_of(i64) array len = 8
+align_of(f64) = 8
+[field_count(S) + 1] len = 4