ComparerCollection: clarify the collection abstraction

[ecomodeller]

While reviewing ComparerCollection, a few things make the "collection" part less clear than it could be. None are urgent; filing for later.

1. __iter__ violates the Mapping contract

ComparerCollection inherits from collections.abc.Mapping, but:

def __iter__(self) -> Iterator[Comparer]:
    return iter(self._comparers.values())

Mapping.__iter__ should yield keys. Because the Mapping mixin builds keys(), items(), and __contains__ on top of __iter__, the consequences are:

• list(cc) → list of Comparer, not names
• cc.keys() → yields Comparer (KeysView delegates to __iter__)
• cc.items() → (Comparer, Comparer) pairs
• dict(cc) is broken

Either drop the Mapping base (it's a custom name-keyed container that happens to be dict-like) or fix __iter__ to yield names and update callers.

2. Container vs. aggregator are conflated

The class fuses two responsibilities:

• Container: storage, indexing, iteration, sel/query/rename fan-out, merge, __getitem__ overloads, zip-based save/load.
• Aggregator: skill(observed=...), mean_skill, weighted score, gridded_skill, plus the supporting machinery (_append_xy_to_res, _attrs_keys_in_by, _add_as_col_if_not_in_index, _mean_skill_by).

They don't need to be separate classes, but splitting them in the file (or factoring the aggregator into a module of free functions on a collection) would make the responsibilities legible.

3. __getitem__ returns different types based on argument shape

cc["alti"] → Comparer; cc[["alti"]] → ComparerCollection. Overloads document it, but a single-element list looks like it should be symmetric with the scalar form. Worth at least a docstring callout.

4. Unenforced invariants leak into helpers

• _unit_text returns self[0]._unit_text with a comment "it should be the same for all" — the check is commented out.
• _name is hardcoded to "Observations".

Small, but they signal that the collection isn't sure what it represents when children disagree.

5. Empty-drop is implicit and inconsistent across selection methods

• sel drops children with n_points == 0.
• query filters then drops empties.
• rename / filter_by_attrs don't drop.

A reader has to inspect each method to know whether obs_names is preserved.

Suggested first step

Fix the Mapping contract — smallest change, clearest payoff, crispest answer to "what is the collection part."