perf(asr): speed up local Flow dictation and land CLM/keyboard refactor

Reduce perceived latency from key release to final text:
- Adaptive chunking: 2.5s first chunk + 5s follow-ups so short
  utterances start on-device recognition while still recording.
- Session-level ASR warmup and audio-format cache reuse to remove
  per-utterance cold-start of SpeechAnalyzer.
- Mirror live pipelined partials to the keyboard transcript line via
  a new flow.transcriptionPartial App Group key + Darwin ping.

Also commits the accumulated custom language model, Flow session,
keyboard extension restructure, and Xiaomi MiMo provider work in
progress on this branch.

OSGKeyboardShared/Models/FlowUtteranceChunkConfig.swift

@@ -6,8 +6,10 @@
 import Foundation
 
 public struct FlowUtteranceChunkConfig: Sendable, Equatable {
-    /// Target maximum duration per ASR chunk.
-    public let maxChunkDurationSeconds: TimeInterval
+    /// Target duration for the first ASR chunk (starts pipelining early).
+    public let firstChunkDurationSeconds: TimeInterval
+    /// Target duration for later chunks once pipelining is underway.
+    public let subsequentChunkDurationSeconds: TimeInterval
     /// Tail overlap fed into the next chunk for boundary dedup when stitching.
     public let overlapDurationSeconds: TimeInterval
     /// After hitting the max window, wait up to this long for a pause before hard-splitting.
@@ -17,21 +19,52 @@ public struct FlowUtteranceChunkConfig: Sendable, Equatable {
     public let sampleRate: Int
 
     public init(
-        maxChunkDurationSeconds: TimeInterval,
+        firstChunkDurationSeconds: TimeInterval = 2.5,
+        subsequentChunkDurationSeconds: TimeInterval = 5.0,
         overlapDurationSeconds: TimeInterval,
         pauseExtensionMaxSeconds: TimeInterval,
         pauseRMSThreshold: Float,
         sampleRate: Int
     ) {
-        self.maxChunkDurationSeconds = maxChunkDurationSeconds
+        self.firstChunkDurationSeconds = firstChunkDurationSeconds
+        self.subsequentChunkDurationSeconds = subsequentChunkDurationSeconds
         self.overlapDurationSeconds = overlapDurationSeconds
         self.pauseExtensionMaxSeconds = pauseExtensionMaxSeconds
         self.pauseRMSThreshold = pauseRMSThreshold
         self.sampleRate = sampleRate
     }
 
+    /// Uniform chunk size — used by unit tests and legacy call sites.
+    public init(
+        maxChunkDurationSeconds: TimeInterval,
+        overlapDurationSeconds: TimeInterval,
+        pauseExtensionMaxSeconds: TimeInterval,
+        pauseRMSThreshold: Float,
+        sampleRate: Int
+    ) {
+        self.firstChunkDurationSeconds = maxChunkDurationSeconds
+        self.subsequentChunkDurationSeconds = maxChunkDurationSeconds
+        self.overlapDurationSeconds = overlapDurationSeconds
+        self.pauseExtensionMaxSeconds = pauseExtensionMaxSeconds
+        self.pauseRMSThreshold = pauseRMSThreshold
+        self.sampleRate = sampleRate
+    }
+
+    /// Backward-compatible alias for tests that read `maxChunkSamples`.
+    public var maxChunkDurationSeconds: TimeInterval {
+        subsequentChunkDurationSeconds
+    }
+
+    public func maxChunkDurationSeconds(forChunkIndex index: Int) -> TimeInterval {
+        index == 0 ? firstChunkDurationSeconds : subsequentChunkDurationSeconds
+    }
+
+    public func maxChunkSamples(forChunkIndex index: Int) -> Int {
+        Int(maxChunkDurationSeconds(forChunkIndex: index) * Double(sampleRate))
+    }
+
     public var maxChunkSamples: Int {
-        Int(maxChunkDurationSeconds * Double(sampleRate))
+        maxChunkSamples(forChunkIndex: 1)
     }
 
     public var overlapSamples: Int {
@@ -44,7 +77,8 @@ public struct FlowUtteranceChunkConfig: Sendable, Equatable {
 
     /// Default for keyboard Flow utterances (≤ 3 min, pipelined ASR).
     public static let flowDefault = FlowUtteranceChunkConfig(
-        maxChunkDurationSeconds: 30,
+        firstChunkDurationSeconds: 2.5,
+        subsequentChunkDurationSeconds: 5.0,
         overlapDurationSeconds: 0.5,
         pauseExtensionMaxSeconds: 2,
         pauseRMSThreshold: 0.015,