Support budgeted per-source suffixes

[aakashH242]

Summary

• add an optional ContextSource suffix for balanced per-source wrappers
• budget prefixes, retained evidence, suffixes, separators, and omission markers as one rendered context
• keep wrapper text out of ranking, embeddings, per-source counts, and source spans
• preserve plain-string and prefix-only behavior
• document the end-user contract and prepare version 0.6.0

Verification

• 274 non-integration tests passed with 97.06% coverage
• real FastEmbed integration test passed
• Ruff check and format passed
• strict mypy passed
• strict MkDocs build passed
• sdist and wheel build passed
• installed wheel passed metadata inspection, uv pip check, and sync/async API smoke tests
• randomized rendering and brute-force exact-fallback checks passed

Closes #27