feat(netcdf): make the container a mapping, and add the xarray-compatible aliases

docs/migration.md

@@ -1033,6 +1033,30 @@ replace the georeference wholesale.
 
 ### unreleased
 
+**A container now duck-types as a sequence, because it gained `__iter__` and `__len__`.** `NetCDF` had
+neither dunder before, so every `isinstance(x, Iterable)` test answered `False` for one. That flipped:
+
+| expression | before | now |
+|---|---|---|
+| `isinstance(nc, collections.abc.Iterable / Sized / Container)` | `False` | `True` |
+| `isinstance(nc, collections.abc.Mapping / Sequence)` | `False` | `False`, unchanged |
+
+- The case to check is a helper of your own that accepts "anything iterable". It used to reject a container
+  outright and now quietly receives a list of name strings, so a mistake fails later and less clearly than it
+  used to. Gate on `isinstance(x, NetCDF)` first if that matters.
+- **Array coercion now raises, where a container used to answer uselessly.** `np.asarray(nc)` returned a 0-d
+  object array wrapping the container; it now raises `TypeError` pointing at `nc["name"].read_array()`. That is
+  a hard change, but the old value was a box around the object rather than any of its data, so nothing could
+  have been computing with it.
+
+  The refusal exists for the *variable* case, where leaving the coercion alone would have been worse than
+  useless: iterating a variable yields nothing, so `np.mean(nc["t2m"])` would have answered `nan` for a cube of
+  real values. `origin/main` raised there, and so does this — only the exception type differs.
+- `np.array([nc], dtype=object)` still boxes the dataset and is still `(1,)`: that request reads nothing and
+  fabricates nothing, so it is the one `__array__` honours.
+- `bool(nc)` is unaffected: `Dataset.__bool__` still refuses, and takes precedence over `__len__`. Ask
+  `len(nc) == 0` whether a container is empty.
+
 **`to_xarray()` decodes the CF time axis, so the coordinate is `datetime64[ns]` rather than `float64`.**
 Hard change, silent for the read side — nothing raises and nothing warns when the axis decodes. The bridge
 exists to hand you to xarray, and `resample`, `.dt` and `groupby("time.<component>")` all raised on the numeric

docs/reference/netcdf/public-api.md

@@ -1,7 +1,8 @@
 # `NetCDF` — public API
 
-A one-line map of every public member the `NetCDF` class itself defines — 65 in all: 38 methods, 20 properties,
-6 classmethods and 1 staticmethod. For the full signatures, arguments and examples, see the rendered
+A one-line map of every public member the `NetCDF` class itself defines — 77 in all: 43 methods, 27 properties,
+6 classmethods and 1 staticmethod, plus the four mapping dunders (`__getitem__`, `__contains__`, `__iter__`,
+`__len__`). For the full signatures, arguments and examples, see the rendered
 [NetCDF Class](index.md) reference; this page is the index you scan to find the member you want.
 
 `NetCDF` extends `Dataset`, so it also inherits a further 134 public members it does not redefine — band
@@ -30,20 +31,51 @@ Two object shapes share this class, and several members behave differently acros
 
 ## Variables, dimensions and groups
 
-| Member                   | What it does                                                              |
-|--------------------------|---------------------------------------------------------------------------|
-| `variable_names`         | Names of the data variables, excluding dimension coordinate arrays.       |
-| `variables`              | Lazy `{name: subset}` mapping of every data variable.                     |
-| `get_variable()`         | Extracts one variable as a classic-raster `NetCDF` (a Variable).          |
-| `get_variable_names()`   | Deprecated alias for the `variable_names` property.                       |
-| `dimension_names`        | Names of all dimensions, in storage order.                                |
-| `dimension_sizes`        | `{name: size}` for every dimension, read from the multidimensional group. |
-| `get_dimension_values()` | Stored coordinates of any dimension — `level`, `depth`, `member`, `time`. |
-| `group_names`            | Names of the sub-groups in the root group.                                |
-| `get_group()`            | Opens a netCDF-4 sub-group as its own Container, without copying data.    |
-| `is_subset`              | Whether this object is a single-variable subset rather than a Container.  |
-| `is_md_array`            | Whether the dataset was opened in multidimensional mode.                  |
-| `file_name`              | The file path, with any `NETCDF:"path":var` prefix stripped.              |
+| Member                   | What it does                                                                    |
+|--------------------------|---------------------------------------------------------------------------------|
+| `variable_names`         | Names of the data variables, excluding dimension coordinate arrays.             |
+| `variables`              | Lazy `{name: subset}` mapping of every data variable.                           |
+| `get_variable()`         | Extracts one variable as a classic-raster `NetCDF` (a Variable).                |
+| `get_variable_names()`   | Deprecated alias for the `variable_names` property.                             |
+| `nc[name]`               | The variable called `name`; `KeyError` where `get_variable` gives `ValueError`. |
+| `name in nc`             | Whether `name` is one of the data variables.                                    |
+| `iter(nc)` / `len(nc)`   | The data-variable names, and how many there are.                                |
+| `get()`                  | The variable, or a default when the container has no such name.                 |
+| `keys()`                 | The data-variable names, as a fresh list.                                       |
+| `values()`               | Every variable, loading each.                                                   |
+| `items()`                | `(name, variable)` for every variable, loading each.                            |
+| `dimension_names`        | Names of all dimensions, in storage order.                                      |
+| `dimension_sizes`        | `{name: size}` for every dimension, read from the multidimensional group.       |
+| `get_dimension_values()` | Stored coordinates of any dimension — `level`, `depth`, `member`, `time`.       |
+| `group_names`            | Names of the sub-groups in the root group.                                      |
+| `get_group()`            | Opens a netCDF-4 sub-group as its own Container, without copying data.          |
+| `is_subset`              | Whether this object is a single-variable subset rather than a Container.        |
+| `is_md_array`            | Whether the dataset was opened in multidimensional mode.                        |
+| `file_name`              | The file path, with any `NETCDF:"path":var` prefix stripped.                    |
+
+## xarray-compatible spellings
+
+Aliases so habits from xarray transfer without renaming anything. Each names its canonical member, and each
+docstring states where it diverges from the xarray member it echoes.
+
+| Member      | What it does                                                                           |
+|-------------|----------------------------------------------------------------------------------------|
+| `data_vars` | `variables` under xarray's name — a mapping, so `nc.data_vars["t2m"]` works.           |
+| `dims`      | `{name: length}` — **not** `dimension_names`, a list. `sizes` is the durable spelling. |
+| `sizes`     | The same mapping as `dims`; xarray is turning its own `dims` into a set of names.      |
+| `attrs`     | `global_attributes` under xarray's name.                                               |
+| `coords`    | `{name: stored coordinate}` for every indexed dimension, from `get_dimension_values`.  |
+
+## Cheap introspection
+
+Metadata only: no data variable's array is read, though opening a variable does read its coordinate axes.
+A variable with no raster plane is the exception — see each member's docstring.
+
+| Member   | What it does                                                                      |
+|----------|-----------------------------------------------------------------------------------|
+| `dtypes` | `{name: dtype}` for every data variable, from the band description.               |
+| `nbytes` | Total size of the data variables, computed from shape and dtype rather than read. |
+| `info()` | Prints an `ncdump -h`-shaped summary to a buffer, or to `sys.stdout`.             |
 
 ## Coordinates and time