feat: IDE支持 — workspace拆分、LSP服务器、VS Code扩展

**Workspace 拆分**
- aster-core: 纯库，zero-dependency，包含 lexer/parser/ast/interpreter/error/analysis
- aster: REPL 二进制，薄封装 aster-core
- aster-lsp: LSP 语言服务器

**VS Code 扩展 (vscode-ext/)**
- TextMate 语法高亮 (.ast 文件)
- 语言配置 (注释切换、括号配对、自动缩进)
- LSP 客户端 (extension.js)

**LSP 服务端功能**
- Diagnostics: 实时显示 lex/parse 错误红色波浪线
- Completion: 关键字 + 内置函数 + 用户定义符号补全
- Hover: 悬停显示变量/函数信息
- Signature Help: 函数参数提示
- Goto Definition: Ctrl+Click 跳转到声明处
- Find References: 查找所有引用位置
- Rename: F2 重命名符号

**新增 analysis 模块**
- collect_symbols: AST 遍历收集符号
- find_declaration/find_all_references: Token 扫描定位声明和引用

aster-lsp/src/hover.rs

@@ -0,0 +1,104 @@
+use std::collections::HashMap;
+
+use aster_core::analysis::{self, SymbolKind};
+use aster_core::lexer::Lexer;
+use aster_core::parser::Parser;
+use lsp_server::{Connection, Message, Request};
+use lsp_types::{HoverContents, MarkupContent, MarkupKind, TextDocumentPositionParams, Uri};
+
+use crate::completion;
+
+const BUILTINS: &[&str] = &[
+    "print", "input", "clock", "len", "typeof", "push", "pop",
+    "split", "trim", "substring", "replace", "contains",
+    "upper", "lower", "starts_with", "ends_with", "require",
+];
+
+pub fn handle_hover(
+    documents: &HashMap<Uri, String>,
+    connection: &Connection,
+    req: Request,
+) {
+    let id = req.id;
+    let params: TextDocumentPositionParams = match serde_json::from_value(req.params) {
+        Ok(p) => p,
+        Err(e) => {
+            eprintln!("Invalid hover params: {}", e);
+            return;
+        }
+    };
+
+    let uri = params.text_document.uri;
+    let position = params.position;
+
+    let text = documents.get(&uri).map(|s| s.as_str()).unwrap_or("");
+    let word = completion::get_word_at_position(text, position);
+
+    if word.is_empty() {
+        let resp = lsp_server::Response::new_ok::<Option<()>>(id, None);
+        let _ = connection.sender.send(Message::Response(resp));
+        return;
+    }
+
+    // Check builtins
+    if BUILTINS.contains(&word.as_str()) {
+        let contents = HoverContents::Markup(MarkupContent {
+            kind: MarkupKind::Markdown,
+            value: format!("**builtin** `{}`", word),
+        });
+        let hover = lsp_types::Hover {
+            contents,
+            range: None,
+        };
+        let resp = lsp_server::Response::new_ok(id, hover);
+        let _ = connection.sender.send(Message::Response(resp));
+        return;
+    }
+
+    // Check keywords
+    const KEYWORDS: &[&str] = &[
+        "let", "const", "fn", "if", "else", "while", "for", "in",
+        "break", "continue", "return", "true", "false", "nil",
+    ];
+    if KEYWORDS.contains(&word.as_str()) {
+        let contents = HoverContents::Markup(MarkupContent {
+            kind: MarkupKind::Markdown,
+            value: format!("**keyword** `{}`", word),
+        });
+        let hover = lsp_types::Hover { contents, range: None };
+        let resp = lsp_server::Response::new_ok(id, hover);
+        let _ = connection.sender.send(Message::Response(resp));
+        return;
+    }
+
+    // Check user-defined symbols
+    let (tokens, _) = Lexer::new(text).tokenize();
+    let mut parser = Parser::new(tokens);
+    let (stmts, _) = parser.parse();
+    let symbols = analysis::collect_symbols(&stmts);
+
+    for sym in &symbols {
+        if sym.name == word {
+            let markdown = match &sym.kind {
+                SymbolKind::Variable => {
+                    format!("**variable** `{}`", sym.name)
+                }
+                SymbolKind::Function { params } => {
+                    format!("**fn** `{}({})`", sym.name, params.join(", "))
+                }
+            };
+            let contents = HoverContents::Markup(MarkupContent {
+                kind: MarkupKind::Markdown,
+                value: markdown,
+            });
+            let hover = lsp_types::Hover { contents, range: None };
+            let resp = lsp_server::Response::new_ok(id, hover);
+            let _ = connection.sender.send(Message::Response(resp));
+            return;
+        }
+    }
+
+    // Not found — return null
+    let resp = lsp_server::Response::new_ok::<Option<()>>(id, None);
+    let _ = connection.sender.send(Message::Response(resp));
+}