fix: dispatch spell-check completion to main queue

NSSpellChecker.requestChecking calls back on NSTextCheckingOperationQueue
(a background queue), not the main thread. The completion handler was
setting textView.needsDisplay = true directly, which:

1. Triggered Main Thread Checker (UI API on background thread)
2. Silently failed to redraw, so underlines never reappeared after edits

Wrap updateSpellMisspelledRanges in DispatchQueue.main.async. The
filtering (pure computation over results) stays on the background queue;
only the cache assignment + needsDisplay hop to main.

Author: Ender-Wang

.tmp/commit-msg.txt

@@ -0,0 +1,24 @@
+feat: stamp .spellingState: 0 on fenced code blocks and inline `code`
+
+Code is not prose. Fenced blocks and inline `code` spans should be left
+alone by the system spell-checker, the same way links, wiki-links, LaTeX,
+and table cells already are. This completes the engine's existing
+spell-check suppression convention with two minimal styler hunks.
+
+- MarkdownASTStyler.styleCodeBlock: append .spellingState: 0 on the
+  whole fenced codeRange (alongside the existing .font/.background/
+  .paragraphStyle stamp).
+- MarkdownASTStyler inline .code case: append .spellingState: 0 on the
+  full inline span (markers + content), so the suppression covers the
+  backticks too.
+- MarkdownASTStylerTests: new @Test asserting .spellingState: 0 is
+  present on a fenced-block content range and on an inline `code` span,
+  and that plain prose carries no .spellingState attribute.
+- CHANGELOG: Unreleased / Added entry framed as completing the existing
+  suppression convention.
+
+This is the styler portion split out of #59 per review feedback. The
+render-pass approach in that PR is being abandoned — the harness
+attached to #59 confirms the underline absence on macOS 15.7.7 is a
+system-wide TextKit 2 issue, not something the custom NSTextLayoutFragment
+subclass causes.

.tmp/spellcheck-pipeline-test.swift

@@ -0,0 +1,100 @@
+// Two-arm test:
+//   1. Does AppKit's continuous spell-check pass WRITE .spellingState to the
+//      NSTextLayoutManager on macOS 15.7.7? We read renderingAttributes back.
+//   2. If we MANUALLY write .spellingState via addRenderingAttribute, does
+//      the underline appear? This is the viability test for EdgeMark's
+//      Option A (manual driver) regardless of question 1.
+import AppKit
+
+let app = NSApplication.shared
+app.setActivationPolicy(.accessory)
+
+func analyze(_ rep: NSBitmapImageRep, label: String) {
+    var red = 0
+    for y in 0..<rep.pixelsHigh {
+        for x in 0..<rep.pixelsWide {
+            guard let c = rep.colorAt(x: x, y: y)?.usingColorSpace(.sRGB) else { continue }
+            let r = c.redComponent, g = c.greenComponent, b = c.blueComponent
+            if r > 0.55 && g < 0.5 && b < 0.5 { red += 1 }
+        }
+    }
+    print("\(label): red=\(red)")
+}
+
+func countSystemSpellingStates(_ tlm: NSTextLayoutManager, label: String) {
+    let documentRange = tlm.documentRange
+    var states: [(NSRange, Int)] = []
+    tlm.enumerateRenderingAttributes(from: documentRange.location, reverse: false) { (tlm2, attrs, range) -> Bool in
+        if let s = attrs[.spellingState] as? Int {
+            let loc = tlm2.offset(from: documentRange.location, to: range.location)
+            let len = tlm2.offset(from: range.location, to: range.endLocation)
+            let nsRange = NSRange(location: loc, length: len)
+            states.append((nsRange, s))
+        }
+        return true
+    }
+    print("\(label): system-wrote .spellingState entries = \(states.count)")
+    for (r, v) in states { print("  range=\(r.location)+\(r.length) value=\(v)") }
+}
+
+func run(label: String, insertManual: Bool) {
+    let storage = NSTextContentStorage()
+    let tlm = NSTextLayoutManager()
+    let container = NSTextContainer(size: CGSize(width: 400, height: 200))
+    tlm.textContainer = container
+    storage.addTextLayoutManager(tlm)
+
+    let tv = NSTextView(frame: NSRect(x: 0, y: 0, width: 400, height: 200), textContainer: container)
+    tv.isEditable = true
+    tv.isContinuousSpellCheckingEnabled = true
+    tv.backgroundColor = .white
+    tv.textColor = .black
+    tv.font = NSFont.systemFont(ofSize: 16)
+
+    let window = NSWindow(contentRect: NSRect(x: 120, y: 120, width: 400, height: 200),
+                          styleMask: [.titled], backing: .buffered, defer: false)
+    window.contentView = tv
+    window.makeKeyAndOrderFront(nil)
+    window.makeFirstResponder(tv)
+    tv.insertText("helllo wrold misspeled", replacementRange: NSRange(location: 0, length: 0))
+
+    // Give continuous spell-check time to run.
+    RunLoop.current.run(until: Date().addingTimeInterval(3.0))
+    tv.needsDisplay = true
+    tv.displayIfNeeded()
+    RunLoop.current.run(until: Date().addingTimeInterval(0.5))
+
+    // Question 1: did the system write .spellingState?
+    countSystemSpellingStates(tlm, label: "\(label) [after system pass]")
+
+    // Question 2: manually write .spellingState on the first misspelled word.
+    if insertManual {
+        let docRange = tlm.documentRange
+        // "helllo" is at offset 0, length 6.
+        let endLoc = tlm.location(docRange.location, offsetBy: 6)!
+        let manualRange = NSTextRange(location: docRange.location, end: endLoc)!
+        tlm.addRenderingAttribute(.spellingState, value: 1, for: manualRange)
+        print("\(label): manually wrote .spellingState:1 on 0..<6")
+    }
+    RunLoop.current.run(until: Date().addingTimeInterval(0.5))
+    tv.needsDisplay = true
+    tv.displayIfNeeded()
+    RunLoop.current.run(until: Date().addingTimeInterval(0.5))
+
+    guard let rep = tv.bitmapImageRepForCachingDisplay(in: tv.bounds) else {
+        print("\(label): no bitmap")
+        window.orderOut(nil)
+        return
+    }
+    tv.cacheDisplay(in: tv.bounds, to: rep)
+    analyze(rep, label: label)
+    if let png = rep.representation(using: .png, properties: [:]) {
+        try? png.write(to: URL(fileURLWithPath: "/tmp/spellcheck_\(label).png"))
+    }
+    window.orderOut(nil)
+}
+
+// Arm A: system pass only (question 1 only)
+run(label: "A_systemOnly", insertManual: false)
+// Arm B: system pass + manual write (questions 1 + 2)
+run(label: "B_withManual", insertManual: true)

Sources/MarkdownEngine/TextView/Coordinator/NativeTextViewCoordinator+SpellCheck.swift

@@ -101,7 +101,11 @@ extension NativeTextViewCoordinator {
                         ? nil
                         : result.range
                 }
-                self.updateSpellMisspelledRanges(filtered, textView: textView)
+                // NSSpellChecker calls back on NSTextCheckingOperationQueue
+                // (a background queue). Hop to main before touching UI.
+                DispatchQueue.main.async {
+                    self.updateSpellMisspelledRanges(filtered, textView: textView)
+                }
             }
         )
     }