fix: use UTF-16 positions in LSP

[vycdev2]

Summary

• convert incoming LSP UTF-16 positions to the driver's Unicode-scalar columns for hover, definition, and completion queries
• convert compiler byte spans to UTF-16 ranges for diagnostics and definition responses
• advertise UTF-16 position encoding and cover supplementary characters, invalid surrogate positions, CRLF, and bounds

Verification

• cargo test -q -p jett_lsp — passed: 5 tests
• cargo build — passed
• cargo test -q — passed: full workspace suite, including 432 driver fixtures
• cargo fmt --check — passed
• git diff --check — passed

Risk

• Low: the change is isolated to the LSP adapter's coordinate boundary and invalid client positions return no result rather than being misinterpreted.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.