fix(embedding): reject non-progressing chunk windows

[deepujain]

Summary

• reject zero-sized embedding chunk windows
• reject negative overlap and overlap greater than or equal to the chunk size
• fail before entering the fixed-window splitter or contacting the embedding provider
• add focused regression coverage for zero and non-progressing windows

Closes #139.

Verification

• .venv/bin/python -m pytest -q tests/embedding/test_client.py — 31 passed
• make PYTHON=.venv/bin/python lint — passed
• .venv/bin/python -m build — sdist and wheel built successfully
• git diff --check origin/main...HEAD — passed
• make PYTHON=.venv/bin/python test — 6,054 passed, 18 skipped, 4 deselected, 4 unrelated baseline failures: one CLI golden-surface drift and three Anthropic SDK httpx2 compatibility expectations

Release Impact

Embedding chunk configuration now fails with an actionable SimilarityConfigError instead of allowing a zero or negative step to stall the fixed-size splitter. The change is backward compatible for valid chunk settings.

[rng1995]

@deepujain Please resolve merge conflicts