Imported methods always had IsOverload=False: the .bif method layouts
round-tripped IsVirtual/IsOverride/IsStatic but not IsOverload, and the
importers never set it. ResolveMethodOverload's hiding walk stops at the
first non-overload candidate, so an overload set split across an imported
class and an imported ancestor would be truncated to the more-derived
level (latent today because the known overload sets live on a single
class).
Add IsOverload to both method-encoding paths and bump BLAISE-IFACE 5 -> 6:
* TRoutineSig (class methods): field added in uUnitInterface, set in
BuildRoutineSig, encoded/decoded in EncodeMethodSig/ReadMethodSig,
propagated in SynthesiseMethodDecl.
* TMethodDecl (interface and generic-template methods, the
EncodeMethodDecl/ReadMethodDecl path): the same flag was dropped there
too, so an overloaded interface method lost its directive across the
.bif.
Round-trip tests for both paths: TestRoundTrip_Class_WithOverloadedMethods
and TestRoundTrip_Interface_WithOverloadedMethods. bif-coverage status
gains TRoutineSig.IsOverload.
a934d73566