Expose SymbolKind (Function, Object, Section, Unknown) in the
DiffSymbol proto message, allowing CLI oneshot diff consumers to
distinguish symbol types without inferring from instruction/data
presence.

…essage

Remove the symbol filter (size == 0 / Ignored) from DiffObject so all
symbols are emitted. This fixes target_symbol indices being invalid
after filtering.

Replace the DiffSymbolFlag bitmask enum + uint32 field with a
DiffSymbolFlags message of booleans, adding ignored and size_inferred
flags so consumers can filter on their end.