M5: LSP pool and diagnostics wiring

Adds the DiagnosticsSource seam trait in harness-core::lsp (kept out of harness-tools so tools never link the LSP crate directly), and implements it in harness-lsp as a pool that lazily spawns one language server per file extension (rust-analyzer, typescript-language-server, gopls, pyright-langserver) only when the binary is on PATH. Adds harness-tools::diagnostics and wires diagnostics into edit/write/bash/glob/grep output.

crates/harness-lsp/tests/rust_analyzer.rs

@@ -0,0 +1,34 @@
+//! End-to-end LSP test against a real `rust-analyzer`. Ignored by default (requires the binary
+//! on PATH and is slow — RA indexes the project). Run with:
+//!   cargo test -p harness-lsp --test rust_analyzer -- --ignored --nocapture
+
+use std::time::Duration;
+
+use harness_core::lsp::{DiagnosticsSource, Severity};
+use harness_lsp::LspPool;
+
+#[tokio::test]
+#[ignore = "requires rust-analyzer on PATH; slow"]
+async fn rust_analyzer_reports_a_type_error() {
+    // Minimal cargo project with a deliberate type error.
+    let dir = tempfile::tempdir().unwrap();
+    std::fs::write(
+        dir.path().join("Cargo.toml"),
+        "[package]\nname = \"probe\"\nversion = \"0.1.0\"\nedition = \"2021\"\n",
+    )
+    .unwrap();
+    std::fs::create_dir_all(dir.path().join("src")).unwrap();
+    let main_rs = dir.path().join("src/main.rs");
+    std::fs::write(&main_rs, "fn main() {\n    let x: i32 = \"not an integer\";\n    let _ = x;\n}\n").unwrap();
+
+    let pool = LspPool::new(dir.path().to_path_buf(), vec![]);
+    pool.touch(&main_rs).await;
+    // Generous wait: rust-analyzer must index the workspace before it reports anything.
+    let diags = pool.diagnostics(&main_rs, Duration::from_secs(60)).await;
+
+    println!("diagnostics: {diags:#?}");
+    assert!(
+        diags.iter().any(|d| d.severity == Severity::Error),
+        "expected at least one error diagnostic, got: {diags:?}"
+    );
+}