Fix MEI importer silently dropping <bTrem> and shifting subsequent onsets (AI)

[youdie006]

Fixes #1994

Problem

music21.mei.base._processEmbeddedElements dispatches each child element through a per-container tag->function mapping. A child whose tag is not in the mapping is silently dropped and never recursed into (only a debug message is emitted). <bTrem> (measured / bowed tremolo, which wraps the <note>/<chord> carrying the notated duration) was registered in none of the dispatch dicts, so the tremolo note/chord and its duration disappeared and every later onset in the layer shifted earlier -- data corruption, not a cosmetic loss. As @weselyj reported, one dropped tremolo chord shifted the remainder of a real piece by a quarter note.

Fix

• Add bTremFromElement, mirroring beamFromElement/tupletFromElement: it imports the wrapped <note>/<chord> with its notated duration and returns it, so timing is preserved. When @unitdur is present it attaches an expressions.Tremolo with numberOfMarks derived from the unit duration (eighth->1 mark, sixteenth->2, ...).
• Register <bTrem> in the layer, beam, and tuplet dispatch tables.
• Scope limited to <bTrem>; the two-note <fTrem> is intentionally out of scope.
• Docstring "Implemented"/"not Implemented" element lists updated to match.

Tests

Added focused tests in music21/mei/test_base.py using the reporter's snippet: the <bTrem> chord now imports on beat 2 with its correct duration, subsequent onsets no longer shift (3 notes at offsets 0.0/1.0/2.0, ql 1/1/2), and the Tremolo expression is attached with the right number of marks. Red-green verified (without the fix the layer imports only 2 notes); the full music21/mei/test_base.py module and the module doctests pass with no regressions; ruff, pylint, and mypy are clean.

Thanks to @weselyj for the precise repro and to @mscuthbert for inviting the PR.

AI disclosure

This contribution was prepared with AI assistance; I take responsibility for its correctness and have verified it against the project's test suite and style checks.

[coveralls]

coverage: 93.329% (+0.003%) from 93.326% — youdie006:fix/1994-mei-btrem-import into cuthbertLab:master