LazyNicheProbe

REGRESSION FIXTURE for the lazy-load niche codegen path.

Intentionally NOT registered in bin/main.ml's stdlib_file_list, so this module is loaded LAZILY (signatures only, no body type inference) when referenced. Its niche-encoded Option match (List.nth_opt) therefore reaches codegen with an unresolved scrutinee type ('_). emit_case must recover the niche shape from the pattern constructors instead of defaulting to the heap-tag match strategy (which dereferences a tagged int → segfault).

Do NOT add this file to stdlib_file_list — that would make it eagerly typechecked and defeat the regression it guards (see test/native/lazy_niche).