chore(mp3): 固定版与 Helix 版的工作中剩余的内容 — — 记账路径过滤器、哈夫曼回归、上游化

Follow-up to #4138 and #4139, which are closed: the fixed-point decoder went from 3.99x Helix on an ESP32-C6 to 1.13x (#4140, #4150), and the shipping backend is now gated on a Callgrind, codegen and exact-operation baseline in `codec_cpu_trend.json` (#4148). Three items from those issues were never done and are collected here so they do not get lost.