feat: migrate on-device Qwen3 ASR to CoreML for background Flow dictation

Replace MLX GPU inference with CoreML bundles so transcription continues
while the host app is backgrounded. Adds model download and warm-up,
vendored Qwen3Speech, and updates onboarding, settings, and copy for the
~1.6 GB CoreML package (iOS 18+).

OSGKeyboardShared/Utilities/ProgressiveDictationTranscriptAccumulator.swift

@@ -0,0 +1,64 @@
+// ProgressiveDictationTranscriptAccumulator.swift
+// OSGKeyboard · Shared
+//
+// Merges progressive `DictationTranscriber` results into one transcript.
+// Short-form presets may emit a new time range after ~30 s; treating the
+// latest partial as the full transcript drops earlier segments.
+
+import Foundation
+import CoreMedia
+
+/// Combines volatile partials and finalized segments from
+/// `DictationTranscriber.results` into a single growing transcript.
+public struct ProgressiveDictationTranscriptAccumulator: Sendable {
+
+    private struct Segment: Sendable {
+        let startSeconds: Double
+        var text: String
+    }
+
+    private var segments: [Segment] = []
+    private var lastEmitted = ""
+
+    public init() {}
+
+    /// Ingest one analyzer result. Returns a non-nil full transcript when the
+    /// composed text changed since the previous emission.
+    public mutating func ingest(range: CMTimeRange, text: String) -> String? {
+        let trimmed = text.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !trimmed.isEmpty else { return nil }
+
+        let start = range.start.seconds
+
+        if let idx = segments.lastIndex(where: { abs($0.startSeconds - start) < 0.001 }) {
+            // Same audio window — volatile refinement of the current segment.
+            segments[idx].text = trimmed
+        } else if let last = segments.last,
+                  trimmed.hasPrefix(last.text) || last.text.hasPrefix(trimmed) {
+            // Cumulative progressive update without a range change.
+            let longer = trimmed.count >= last.text.count ? trimmed : last.text
+            segments[segments.count - 1].text = longer
+        } else {
+            // New time range — append instead of replacing earlier speech.
+            segments.append(Segment(startSeconds: start, text: trimmed))
+        }
+
+        let full = composedText()
+        guard full != lastEmitted else { return nil }
+        lastEmitted = full
+        return full
+    }
+
+    /// Final composed transcript after the results stream finishes.
+    public mutating func finalize() -> String {
+        let full = composedText()
+        lastEmitted = full
+        return full
+    }
+
+    private func composedText() -> String {
+        segments.reduce(into: "") { partial, segment in
+            partial = DictationTextComposer.compose(anchor: partial, live: segment.text)
+        }
+    }
+}