Add budgeted wrappers to shared context output

[aakashH242]

Summary

• add frozen ContextSource(text, prefix="") inputs for output-only source prefixes
• add an optional separator to trim_context() and atrim_context()
• return exactly measured prompt-ready text for wrapper-aware calls while preserving legacy plain-string accounting
• keep prefixes and separators out of segmentation, ranking, embedding callbacks, and source spans
• prepare the additive API as v0.5.0 and update end-user documentation and wheel smoke coverage

Classification and root cause

This is an enhancement, not a regression. The v0.4 context API deliberately measured only returned source rows; labels and separators added afterward were outside its limit. Conditional prefixes cannot be reserved accurately as one fixed allowance because they cost space only when their source contributes. This change charges that cost during selection and remeasures the complete rendered string.

Verification

• 256 non-integration tests passed with 97.24% coverage
• 1 real CPU FastEmbed integration test passed
• Ruff check and format passed
• strict mypy passed
• strict MkDocs build passed
• 1,600 randomized wrapper/custom-counter cases passed
• clean v0.5.0 sdist and wheel built
• clean Python 3.12 wheel install, pip check, API smoke, metadata, and embedded README checks passed

Closes #25