fix: suppress stale LSP diagnostics

[vycdev2]

Summary

• Track the latest LSP document version alongside each open document.
• Publish diagnostics only when validation still matches the document version currently stored for that URI.
• Keep hover, definition, and completion queries reading the document text from the versioned store.

This prevents an older, slower validation from overwriting diagnostics produced for a newer edit.

Verification

• cargo fmt --all -- --check — passed.
• cargo check --workspace — passed.
• cargo build --workspace — passed.
• cargo test -q -p jett_lsp — passed (4 tests).
• cargo test -q — passed, including 432 driver fixtures.
• git diff --check — passed.
• cargo clippy -q -p jett_lsp --all-targets -- -D warnings — blocked by pre-existing warnings in jett_lexer, jett_fmt, and jett_parser; none are in this diff. The lexer warnings are covered by the existing open lexer PR chore: modernize lexer predicates #120.

Risk

• Low: this only suppresses obsolete diagnostic publications; diagnostics for the current document version continue to publish normally.

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

[vycdev]

Stale-version diagnostic suppression is correct in isolation and the LSP tests passed, but this branch now conflicts with current main after #88 landed. Please rebase while preserving the UTF-16 position conversions, avoid losing the versioned document-state behavior, and rerun the full jett_lsp suite. #116 and #126 are complementary and should be rebased/landed serially.