fix: RNTuple footer leak on extend, broken WritableNTuple.compression, directory __delitem__ KeyError

[henryiii]

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What was broken

1. RNTuple extend() leaked footer-sized regions into FreeSegments on every call

NTuple.extend in _cascadentuple.py called freesegments.release(start, start + old_footer_key.allocation), but Key.allocation is just Key.num_bytes (the ~70-byte key record header). The full allocated region is num_bytes + compressed_bytes. Every call to extend() left the footer payload stranded in FreeSegments, causing permanent file bloat proportional to the number of extends. This parallels the TTree path in _cascadetree.py:487 which correctly uses num_bytes + compressed_bytes.

Reproducer: Create an RNTuple and call extend() 10 times — the file is larger than it needs to be and FreeSegments entries accumulate. After the fix the freed region is fully reclaimed.

2. WritableNTuple.compression getter/setter raised AttributeError

The property was copy-pasted verbatim from WritableTree but NTuple has no _branch_data; it always raised AttributeError: 'NTuple' object has no attribute '_branch_data'. The getter now returns the actual file-level compression from self._cascading._freesegments.fileheader.compression. The setter raises NotImplementedError with a clear message since per-RNTuple compression is not currently supported by the cascading implementation.

Reproducer:

with uproot.recreate("f.root") as f:
    f['t'] = {'x': np.array([1.0], 'f4')}
    print(f['t'].compression)  # was AttributeError, now returns ZLIB(1)

3. WritableDirectory.__delitem__ raised AttributeError for nonexistent keys

_del called key.seek_location without checking whether get_key() returned None. Deleting a nonexistent key raised AttributeError: 'NoneType' object has no attribute 'seek_location' instead of a KeyError subclass as the MutableMapping contract requires. Now raises uproot.exceptions.KeyInFileError (a KeyError subclass), exactly like _get.

Reproducer:

with uproot.recreate("f.root") as f:
    f['t'] = {'x': np.array([1], 'i4')}
    del f['nonexistent']  # was AttributeError, now KeyInFileError

4. Dead/broken code removed

• Ntuple_PageLink class: unreferenced anywhere; its serialize packed two values into a single-field struct → guaranteed struct.error.
• NTuple.branch_types and NTuple.field_name properties: referenced self._branch_types / self._field_name which are never set on NTuple → guaranteed AttributeError.
• NTuple_Footer.__repr__: referenced nonexistent self.metadata_block_envelope_links → guaranteed AttributeError.
• NTuple.write_updates: a one-line stub (sink.flush()) that is never called from anywhere.
• Tree.branch_types in _cascadetree.py: referenced self._branch_types which is never set (branch types are stored in _branch_data).

What changed

• src/uproot/writing/_cascadentuple.py: fix footer release extent; remove 5 dead/broken code items
• src/uproot/writing/_cascadetree.py: remove Tree.branch_types dead property
• src/uproot/writing/writable.py: fix WritableNTuple.compression; fix WritableDirectory._del null check

Skipped

Nothing was skipped. All 4 categories of findings were verified and fixed.

Test results

29 tests passed, 6 skipped (ROOT not installed) across the relevant test files.

Part of #1646.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.86%. Comparing base (1c06db0) to head (da0aef6).
⚠️ Report is 16 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/uproot/writing/_cascadentuple.py	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (80.00%) is below the target coverage (98.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/uproot/writing/_cascadetree.py	83.78% <ø> (+0.07%)	⬆️
src/uproot/writing/writable.py	80.10% <100.00%> (+2.11%)	⬆️
src/uproot/writing/_cascadentuple.py	88.96% <0.00%> (+0.99%)	⬆️

... and 1 file with indirect coverage changes