feat(snowflake-core): implement core text editing primitives

Add comprehensive text editing foundation with:
- Position: line/column with LSP conversion and ordering
- Range: start/end positions with contains, intersects, is_empty
- Buffer: ropey wrapper for efficient insert/delete/replace
- Selection: anchor/head model with direction tracking
- Edit/EditHistory: undo/redo with 1000 entry stack
- Document: file I/O, language detection, full editing API

Includes 67 tests covering edge cases:
- Multiple undo/redo cycles
- Multiline operations
- Unicode handling
- Out-of-bounds clamping
- Empty operations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: fazxes

Cargo.lock

@@ -14,3 +14,5 @@ workspace = true
 serde = { workspace = true }
 serde_json = { workspace = true }
 thiserror = { workspace = true }
+ropey = "1.6"
+lsp-types = { workspace = true }

crates/snowflake-core/Cargo.toml

@@ -1,9 +1,23 @@
 //! Snowflake Core - Shared types and utilities
 //!
 //! This crate provides the foundation types used across all Snowflake crates.
+//!
+//! # Text Editing
+//!
+//! The [`text`] module provides core text editing primitives:
+//!
+//! - [`text::Position`] - A position in a document (line, column)
+//! - [`text::Range`] - A range between two positions
+//! - [`text::Buffer`] - An efficient text buffer backed by a rope
+//! - [`text::Selection`] - A selection with anchor and head
+//! - [`text::Edit`] - An edit operation
+//! - [`text::EditHistory`] - Undo/redo history
+//! - [`text::Document`] - A text document with editing capabilities
 
 pub mod error;
+pub mod text;
 pub mod types;
 
 pub use error::{Error, Result};
+pub use text::{Buffer, Document, DocumentMeta, Edit, EditHistory, Selection, SelectionDirection};
 pub use types::*;