fix: two delegation bugs in dol's own code (content_url, filesys key validation)

[thorwhalen]

Two live bugs in dol itself, both instances of #83's root cause: a wrapper delegates a
non-dunder method to an inner layer bound to that layer, so it receives the outer, unmapped
key.

1. content_url returns a URL for the wrong object

>>> class Served(dict):
...     def url_for(self, key): return f'https://cdn/{key}'
>>> wrapped = KeyCodecs.prefixed('a/')(Served)({'a/k': b'v'})
>>> wrapped['k']
b'v'                          # reads a/k
>>> content_url(wrapped, 'k')
'https://cdn/k'               # WRONG -- addresses k, not a/k

content_url did a flat getattr(store, 'url_for')(key). It now walks the .store chain to
the layer that actually defines url_for, applying each outer layer's _id_of_key on the
way and stopping there — because that layer applies its own.

Stopping matters. Resolving all the way would double-apply the provider's own transform, which
breaks exactly the backend dol/content.py's docstring names — an s3dol store, which owns a
prefix:

content_url(KeyCodecs.prefixed('x/')(Prefixed('logs/')), 'f')
# 'https://s3/logs/x/f'   — not 'https://s3/logs/logs/x/f'

Uses inspect.getattr_static to distinguish a real url_for from a class-wrap's forwarding
DelegatedAttribute — without it the walk stops at the wrapper and never maps the key.

None of the existing tests caught this: they are all identity-keyed.

2. Files(d).is_valid_key(k) is False for a key that exists

>>> s = Files(d)
>>> list(s)
['a.txt']
>>> s.is_valid_key('a.txt')
False                         # WRONG -- and validate_key() raises on it

Files is mk_relative_path_store(FileBytesPersister), so is_valid_key/validate_key were
reached through Store.__getattr__ with the relative key, while the leaf matched them
against a pattern built from the absolute path. This breaks the invariant
all(s.is_valid_key(k) for k in s).

mk_relative_path_store now maps the key before delegating — the same thing its
with_key_validation=True branch (dol/paths.py:1199-1206) already did by hand for
_id_of_key.

focal inherits this via LocalBinaryStore(Files).

Verification

• Full suite: 513 passed, 3 skipped.
• 5 new tests: class-wrap, instance-wrap, Pipe, the no-double-apply case, keyless wraps
  unchanged, and the all(is_valid_key(k) for k in s) invariant.
• Dependents gate byte-for-byte identical to HEAD — xdol 4, pdfdol 23+1 skipped, chromadol
  2, focal 1, dol_cookbook 1, ftpdol 1. mongodol/sqldol/hfdol/dropboxdol/sshdol fail on absent
  backend SDKs both before and after.

Part of #83. Independent of #84 (different modules); either can merge first.