feat: rope-backed Documents store with self.documents() (#12)

[meymchen]

Summary

Closes #12.

The root of the document-sync work (CONTEXT.md, ADR 0003 / 0005): a rope-backed document store, concurrency-safe and reachable from handlers without writing lock code.

Change

• Document — backed by ropey::Rope, keyed by Uri, carrying language id, version, and contents. ropey never leaks into the public API.
• Documents — Arc<RwLock<..>> handle, cheap to clone. Exposed on the trait via LanguageServer::documents() and on the handler receiver via Context::documents(); all clones share one store.
• Position encoding — PositionEncoding field defaulting to Utf16 (the LSP-mandatory default; Position encoding negotiation (UTF-8 preferred, UTF-16 fallback) #10 layers the UTF-8 handshake on top). Both position <-> offset paths read it: UTF-8 native byte index, UTF-16 via char/utf16 helpers (ADR 0016). Position.character is never treated as a ropey char index.
• Mutation primitives — open / apply_incremental_change / close / save as plain methods.

Robustness (from review)

• apply_incremental_change advances the document version, and rejects a reversed range (end before start) with InvalidRequest instead of panicking Rope::remove under the write lock (which would poison the store for every later access).
• save() returns None for a URI that was never opened.
• UTF-8 position_to_offset rejects mid-codepoint and past-end-of-line offsets.

Out of scope

• Wiring the mutations into the dispatcher read-loop and adding the didChange/didClose/didSave trait methods — Apply built-in document-sync mutation inline in the read-loop #9.
• The client-capability UTF-8 negotiation handshake — Position encoding negotiation (UTF-8 preferred, UTF-16 fallback) #10 (this PR only provides the field it writes into).

Test

tests/documents.rs — 16 tests covering read-back, encoding default/set, UTF-8 & UTF-16 conversions (incl. emoji / combining-char round-trips), incremental + full-document replacement with version assertions, reversed-range rejection (store stays usable), save on a missing doc, UTF-8 boundary rejection, cheap clone, and self.documents()/ctx.documents() sharing the same store. cargo clippy --all-targets clean.

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