Preserve ordered keyword expansions and dictionary boundaries

[EiNSTeiN-]

I am opening this PR to preserve duplicate-key checks and evaluation order across multiple keyword expansions. These two functions currently decompile to the same call even though their semantics differ:

def direct(fn, first, second):
    return fn(1, fixed=2, **first, **second)


def dictionary(fn, first, second):
    return fn(1, **{"fixed": 2, **first, **second})

The incorrect output for direct is:

def direct(fn, first, second):
    return fn(1, **{'fixed': 2, **first, **second})

Direct call expansions reject duplicate keywords. Dictionary displays overwrite duplicate keys before the final expansion. Flattening the first call into a single dictionary therefore removes its duplicate-key checks.

That distinction also controls side effects:

fn(1, fixed=2, **get_first(), **get_second())

If get_first() returns a mapping containing fixed, the source raises before evaluating get_second(). The dictionary-display reconstruction evaluates both producers and silently replaces the earlier value.

The decompiler now retains ordered expansion entries for DICT_MERGE calls, keeps DICT_UPDATE dictionary boundaries, and validates named keyword spellings and uniqueness before decomposition. The printer emits ** for unnamed keyword entries. This preserves interleaved forms such as fn(1, **first, fixed=2, **second) as well as intentionally constructed dictionaries with keys such as "my col", "class", or repeated literal keys.

Multiple unpacking begins in Python 3.5. The repaired DICT_MERGE/DICT_UPDATE instruction layout begins in Python 3.9; the older BUILD_MAP_UNPACK_WITH_CALL path is outside this repair.