fix: mk_dirs_if_missing leaves no stray directory for a key a layer below rejects (#122)

[thorwhalen]

Fixes #122. mk_dirs_if_missing(filt_iter(Files, filt=...)): the preset made the directory before the inner filt_iter refused the key. Layers cannot be asked in advance (see #129/#130 for a probe), so the decorator now wraps __setitem__ itself: it makes the missing directories (recording only those this call made), writes, and if the write raises removes them again, deepest first, only if still empty. Other directories, and anything another writer put there, are untouched. mk_dirs_if_missing_preset is kept unchanged (public) but the decorator no longer uses it.

Adversarial review (Opus): no defect in any stacking (class/instance, either side of filt_iter/wrap_kvs/KeyCodecs, doubled, graze/astern suites); found a race where a directory created meanwhile by another process was counted as made here: fixed (level-by-level mkdir, tracked only on success) with a test. Residual, documented: a concurrent writer that had just seen the new directory exist can lose it to the cleanup (its write fails like any write to a missing directory).

Dependents gate (101 suites, timeout 2400 priv test-dependents dol with this branch on PYTHONPATH vs master): identical failing set (25 environmental/pre-existing: missing wkhtmltopdf, couchdb, collection errors...). mergeset passed 320 vs 331 with the same 2 failures (its tree changed between the runs). /dev/null intact.

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