feat(compression): implement stats tracking module

Add estimateCompressionTokens() (char/4 heuristic), createCompressionStats() for per-request stats (original/compressed tokens, savings %, techniques), and trackCompressionStats() for logging.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

open-sse/services/compression/stats.ts

@@ -0,0 +1,38 @@
+import type { CompressionStats, CompressionMode } from "./types";
+
+const CHARS_PER_TOKEN = 4;
+
+export function estimateCompressionTokens(text: string | object | null | undefined): number {
+  if (!text) return 0;
+  const str = typeof text === "string" ? text : JSON.stringify(text);
+  return Math.ceil(str.length / CHARS_PER_TOKEN);
+}
+
+export function createCompressionStats(
+  originalBody: Record<string, unknown>,
+  compressedBody: Record<string, unknown>,
+  mode: CompressionMode,
+  techniquesUsed: string[]
+): CompressionStats {
+  const originalTokens = estimateCompressionTokens(originalBody);
+  const compressedTokens = estimateCompressionTokens(compressedBody);
+  const savingsPercent =
+    originalTokens > 0
+      ? Math.round(((originalTokens - compressedTokens) / originalTokens) * 10000) / 100
+      : 0;
+  return {
+    originalTokens,
+    compressedTokens,
+    savingsPercent,
+    techniquesUsed,
+    mode,
+    timestamp: Date.now(),
+  };
+}
+
+export function trackCompressionStats(stats: CompressionStats): void {
+  if (stats.originalTokens <= 0) return;
+  console.log(
+    `[COMPRESSION] mode=${stats.mode} tokens=${stats.originalTokens}->${stats.compressedTokens} savings=${stats.savingsPercent}% techniques=${stats.techniquesUsed.join(",")}`
+  );
+}