A block-local type is visible only within the source that declares it. The
global `local_type_names` set was source-insensitive, so an imported generic
template's field (resolved in the template's source context, attempt-4) could
bind a type the CALLER declared block-local — silently compiling an undeclared
imported field instead of diagnosing it.
Key `local_type_names` by declaring source. The bare-TYPE gate now resolves a
local only when the query originates in the local's own source (R2 preserved);
a same-name block-local of a DIFFERENT source routes to the undeclared path so
the leak surfaces (`unknown type '...'`, exit 1) instead of escaping via the
`registered` catch-all that would otherwise resolve the globally-registered
cross-source local.
Regression: examples/0762 — imported `Bad :: struct($T) { x: T; y: LocalOnly; }`
with `LocalOnly` declared only in the caller `main` now errors in lib.sx
(fail-before on 8162170 printed `1 9` exit 0).
