feat(lsp+tool-edit-file): multi-server diagnostics + per-edit auto-append

LSP extension:
- Multi-server aggregation: query ALL connected servers matching the
  file's extension (not just the first), merge diagnostics tagged by source
- Incremental sync: capture each server's textDocumentSync.change during
  initialize; compute prefix/suffix diff ranges for change:2 servers;
  full content for change:1 (generic, works for any LSP)
- New diff.ts: pure computeChangeRange + offsetToPosition (O(n), tested)
- Buffer sync: change(filePath, newText) sends didChange with post-edit
  in-memory content; openWithText for first open; tracks open doc text
- languageId mapping: extended with .rb/.rbs/.c/.cpp/etc. (was 'unknown')
- waitForDiagnostics: accepts text override + timeoutMs; returns
  { formatted, slow, timedOut }; polls for publishDiagnostics push
- DiagnosticsStore: hasReceivedPush/clearReceived tracking; formatFiltered
  with minSeverity (1=Error, 2=Warning) for edit_file integration
- LspService.getDiagnostics: service method for cross-extension use

tool-edit-file:
- After successful edit, calls LSP getDiagnostics with post-edit buffer
- Only appends diagnostics with severity ≤ 2 (errors+warnings, no noise)
- Appends slow warning (>10s): 'LSP is taking unusually long...'
- 60s timeout; graceful degradation when no LSP available
- Optional dep on @dispatch/lsp (getService pattern, not manifest depOn)

1468 vitest pass (was 1453, +15 new diff tests).

1 files changed, 117 insertions, 0 deletions

diff --git a/packages/lsp/src/diff.test.ts b/packages/lsp/src/diff.test.ts
new file mode 100644
index 0000000..b5b6a7b
--- /dev/null
+++ b/packages/lsp/src/diff.test.ts
@@ -0,0 +1,117 @@
+import { describe, expect, it } from "vitest";
+import { computeChangeRange, offsetToPosition } from "./diff.js";
+
+describe("offsetToPosition", () => {
+	it("returns 0:0 for offset 0", () => {
+		expect(offsetToPosition("hello", 0)).toEqual({ line: 0, character: 0 });
+	});
+
+	it("counts characters on the first line", () => {
+		expect(offsetToPosition("hello", 3)).toEqual({ line: 0, character: 3 });
+	});
+
+	it("resets character count after newline", () => {
+		expect(offsetToPosition("ab\ncd", 4)).toEqual({ line: 1, character: 1 });
+	});
+
+	it("handles multiple lines", () => {
+		expect(offsetToPosition("a\nb\nc", 4)).toEqual({ line: 2, character: 0 });
+	});
+
+	it("clamps offset beyond text length", () => {
+		expect(offsetToPosition("ab", 100)).toEqual({ line: 0, character: 2 });
+	});
+
+	it("handles empty string", () => {
+		expect(offsetToPosition("", 0)).toEqual({ line: 0, character: 0 });
+	});
+});
+
+describe("computeChangeRange", () => {
+	it("detects a single-line insertion", () => {
+		const oldText = "hello world";
+		const newText = "hello cruel world";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 6 });
+		expect(change.range.end).toEqual({ line: 0, character: 6 });
+		expect(change.text).toBe("cruel ");
+	});
+
+	it("detects a single-line deletion", () => {
+		const oldText = "hello cruel world";
+		const newText = "hello world";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 6 });
+		expect(change.range.end).toEqual({ line: 0, character: 12 });
+		expect(change.text).toBe("");
+	});
+
+	it("detects a single-line replacement", () => {
+		const oldText = "hello world";
+		const newText = "hello earth";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 6 });
+		expect(change.range.end).toEqual({ line: 0, character: 11 });
+		expect(change.text).toBe("earth");
+	});
+
+	it("handles multi-line changes with correct line positions", () => {
+		const oldText = "line1\nline2\nline3";
+		const newText = "line1\nCHANGED\nline3";
+		const change = computeChangeRange(oldText, newText);
+		// Common prefix: "line1\n" → start at beginning of line 1
+		expect(change.range.start).toEqual({ line: 1, character: 0 });
+		// Common suffix: "\nline3" → end after "line2" on line 1
+		expect(change.range.end).toEqual({ line: 1, character: 5 });
+		expect(change.text).toBe("CHANGED");
+	});
+
+	it("handles insertion at end of file", () => {
+		const oldText = "abc";
+		const newText = "abcdef";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 3 });
+		expect(change.range.end).toEqual({ line: 0, character: 3 });
+		expect(change.text).toBe("def");
+	});
+
+	it("handles complete file replacement (no common prefix/suffix)", () => {
+		const oldText = "abc";
+		const newText = "xyz";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 0 });
+		expect(change.range.end).toEqual({ line: 0, character: 3 });
+		expect(change.text).toBe("xyz");
+	});
+
+	it("handles empty old text (new file)", () => {
+		const oldText = "";
+		const newText = "hello\nworld";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 0 });
+		expect(change.range.end).toEqual({ line: 0, character: 0 });
+		expect(change.text).toBe("hello\nworld");
+	});
+
+	it("handles identical text (no change)", () => {
+		const oldText = "same text";
+		const newText = "same text";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 9 });
+		expect(change.range.end).toEqual({ line: 0, character: 9 });
+		expect(change.text).toBe("");
+	});
+
+	it("handles change spanning multiple lines", () => {
+		const oldText = "function foo() {\n  return 1;\n}\n";
+		const newText = "function foo() {\n  return 2;\n  console.log('hi');\n}\n";
+		const change = computeChangeRange(oldText, newText);
+		// Common prefix: "function foo() {\n  return " (26 chars)
+		// The change starts at "1" on line 1, character 9
+		expect(change.range.start).toEqual({ line: 1, character: 9 });
+		// Common suffix: ";\n}\n" (4 chars) → end at offset 27 (the ";")
+		// offset 27 = line 1, char 10 (after "  return 1")
+		expect(change.range.end).toEqual({ line: 1, character: 10 });
+		expect(change.text).toBe("2;\n  console.log('hi')");
+	});
+});