Counter's API beyond the constructor

Split out of [[bug-n-from-collections-import-counter-binds-something-that-always-answers-zero]] (fixed: Counter("aab")["a"] now answers 2, and Counter.update(str) counts characters). While probing the boundary of that fix, three more Counter spellings turned out to be absent. They are filed separately because they are a different thing — the fixed ticket was a silent wrong value, these are missing features that fail loudly, which is the behaviour PyImportIsConsumedOnly's consume-and-ignore rule promises.

Measured at dev 2026-08-26, self-hosted binary at the Counter fix:

spelling	CPython	NilPy today
`Counter({"a": 2, "b": 1})`	works	compile error, listing `Counter()` / `Counter(class)` / `Counter(AnsiString)`
`Counter("aab").elements()`	`['a','a','b']`	`AttributeError: 'TPyDict' object has no attribute 'elements'`
`Counter("aab") - Counter("a")`	`Counter({'a': 1, 'b': 1})`	`TypeError: unsupported operand type(s) for this operator`

Also Counter("aab") + Counter("a"), .subtract(), .total(), and most_common(n) beyond the two arities already present — worth a sweep rather than one addition at a time.

Notes for whoever takes it

TPyDict.update(d: TPyDict) already does Counter-mode addition correctly (it pyvar_to_ints both sides and adds). So Counter(d: TPyDict) is the same three-line delegation the string and list arms now use, and the -/+ operators are a walk over keylist/vallist on top of it. The counting logic exists; what is missing is the surface.
Counter is a mode on TPyDict, not a subclass (see the FCounterMode note in pylib.pas — a subclass loses subscript assignment, bug-pascal-subclass-inherited-members). So elements() and the operators go on TPyDict guarded on the mode, exactly like update and most_common.
CPython's - drops non-positive counts and + keeps only positive ones; subtract() is the one that keeps zero and negative. Easy to get backwards.
Keep one counting loop. The fixed ticket's whole shape was a second copy of the string-counting loop drifting from the first.

Where

compiler/builtin/pylib.pas — the Counter overload set (~2229 interface, ~7685 bodies) and the TPyDict Counter-mode members (FCounterMode, update, most_common).

Gate

Track N: make compiler/pascal26 + a .npy test with a CPython-generated .expected, wired into test-core next to test/test_nilpy_counter_from_a_string.npy.