feat(keyboard): add English QuickType bar and system lexicon

Show verbatim/correction/completion slots, mmap a 40k-word list, and
use UITextChecker plus supplementary lexicon for conservative autocorrect.

OSGKeyboardShared/Typing/EnglishSystemLexicon.swift

@@ -0,0 +1,67 @@
+// EnglishSystemLexicon.swift
+// OSGKeyboard · Shared
+//
+// Apple's sanctioned English sources for a custom keyboard: UITextChecker
+// completions / guesses, plus UILexicon names from
+// `requestSupplementaryLexicon`. The engine stays pure; the keyboard
+// extension fills these fields on each refresh.
+
+import Foundation
+#if canImport(UIKit)
+import UIKit
+#endif
+
+@MainActor
+public protocol EnglishSystemLexiconProviding: AnyObject {
+    func completions(prefix: String, limit: Int) -> [String]
+    func guesses(for word: String, limit: Int) -> [String]
+}
+
+@MainActor
+public final class EmptyEnglishSystemLexicon: EnglishSystemLexiconProviding {
+    public init() {}
+
+    public func completions(prefix: String, limit: Int) -> [String] {
+        []
+    }
+
+    public func guesses(for word: String, limit: Int) -> [String] {
+        []
+    }
+}
+
+#if canImport(UIKit)
+/// System spellchecker. Always called from `TypingSessionController` (@MainActor).
+@MainActor
+public final class UIKitEnglishSystemLexicon: EnglishSystemLexiconProviding {
+    public var language: String
+
+    public init(language: String = "en_US") {
+        self.language = language
+    }
+
+    public func completions(prefix: String, limit: Int) -> [String] {
+        guard !prefix.isEmpty, limit > 0 else { return [] }
+        let checker = UITextChecker()
+        let range = NSRange(location: 0, length: (prefix as NSString).length)
+        let hits = checker.completions(forPartialWordRange: range, in: prefix, language: language) ?? []
+        return Array(hits.prefix(limit))
+    }
+
+    public func guesses(for word: String, limit: Int) -> [String] {
+        guard word.count >= 3, limit > 0 else { return [] }
+        let checker = UITextChecker()
+        let range = NSRange(location: 0, length: (word as NSString).length)
+        let hits = checker.guesses(forWordRange: range, in: word, language: language) ?? []
+        return Array(hits.prefix(limit))
+    }
+
+    public static func learnWord(_ word: String) {
+        let trimmed = word.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !trimmed.isEmpty else { return }
+        if !UITextChecker.hasLearnedWord(trimmed) {
+            UITextChecker.learnWord(trimmed)
+        }
+    }
+}
+#endif