fix(dig): resolution failures raise instead of returning None; export inner_most_key

[thorwhalen]

Three related defects in dol.dig, found while building an S3 adapter that needs to resolve a
key through a wrapped store.

1. inner_most_key returned None on a resolution failure

When no layer of the chain defines _id_of_key, last_element over an empty generator returned
None. Callers use the result as a key or a path, so it surfaced far from its cause:

>>> from dol.dig import inner_most_key
>>> inner_most_key({}, 'some_key')     # before: None

That is what turns a presigned URL into https://.../None, and what makes
MakeMissingDirsStoreMixin die with TypeError: expected str, bytes or os.PathLike object, not NoneType. It now raises AttributeError naming the method and the argument. default= opts
out.

2. store_trans_path ignored its own method argument when recursing

Line 43 hardcoded unravel_key, so _id_of_key was applied at every layer below the first
regardless of what was asked for. This is the acknowledged TODO on inner_most_val — it applied
_data_of_obj at the top layer and _id_of_key below it. Now recurses with the given method:

>>> store = wrap_kvs(wrap_kvs({'k': 1}, data_of_obj=lambda v: v * 10), data_of_obj=lambda v: v + 1)
>>> inner_most_val(store, 5)
60                                      # (5 + 1) * 10 -- before, both layers were not applied

3. inner_most_key / unravel_key are now exported from dol

They were importable only from dol.dig, while being the documented answer to "resolve a key
through a wrap" — which pushed every *dol adapter into a private-module import.

Also: two bugs in MakeMissingDirsStoreMixin.__setitem__

Its recovery path could never have worked. It fed the None from (1) into os.path.dirname,
and then passed keyword-only verbose positionally to ensure_dir. Both raised TypeError, so
the mixin turned a recoverable write error into an unrelated crash. Now resolves with
default=k — correct, because the mixin targets persisters whose keys are full paths and
which therefore define no _id_of_key.

Compatibility

The only behaviour change is (1), and it converts a silent wrong answer into a loud one, in line
with dol's No Silent Failures convention. Ecosystem callers of inner_most_key (xdol,
tapyoca, graze's vendored copy, py2store's star re-export) all pass stores that define
_id_of_key, so none of them reach the new raise. inner_most_val/unravel_val have no
callers outside dol.

Verification

• Full suite: 511 passed, 3 skipped.
• 6 new tests in test_dig.py: the raise, the default= escape, the recursion fix, the export,
  resolution through a real wrap, and a MakeMissingDirsStoreMixin round-trip.
• Dependents gate byte-for-byte identical to HEAD for every dependent whose backend SDK is
  installed here — xdol 4, pdfdol 23+1 skipped, chromadol 2, focal 1, dol_cookbook 1, ftpdol 1.
  mongodol/sqldol/hfdol/dropboxdol/sshdol fail on absent SDKs (pymongo, sqlalchemy, datasets,
  dropbox, paramiko) both before and after.

Needed by the s3dol v1 redesign (i2mint/s3dol#17), which resolves keys through possibly-wrapped
stores and cannot depend on a private module or a silent None.