Add post-processing and integrate fb_power into perf hooks (#500)

Summary:
Pull Request resolved: #500

The diff stack enables power-telemetry tracing for Benchpress plugins and supports the following platforms: **T1_CPL, T1_MLN, T1_BGM, T1_TRN, and T11_GRC_ARM**.

- **Task:** T248954685
- **Claude CLI prompts:**
  - `design.md` (P2203208599)
  - `implementation.md` (P2203210057)
- **Related older diffs:**
  - D73953894
  - D92542871

**Summary**
Adds a **post-processing pipeline** to the `fb_power` monitor and wires it into **perf hooks** so power results can be computed automatically after collection. The post-processing engine reads `power.csv` and emits normalized outputs using **platform-specific sensor-mapping configuration**.

**Changes**
- Add `fb_power/post_process.py` implementing `PowerPostProcessor`
- Add/extend `constants.py` with per-platform sensor-mapping config used by post-processing
- Update `perf.py` to invoke `fb_power` with `post_process` enabled (plumb/forward the flag as needed)
- Update `BUCK` targets to include `post_process.py`
- Export `PowerPostProcessor` from `__init__.py` for external use

Differential Revision: D94542927

Author: lrh12580

benchpress/plugins/hooks/perf.py

@@ -27,7 +27,7 @@
 )
 
 if not open_source:
-    from .perf_monitors.fb_power import monitor as fb_power
+    from .perf_monitors import fb_power
 
 DEFAULT_OPTIONS = {
     "mpstat": {
@@ -48,7 +48,7 @@
 }
 
 if not open_source:
-    DEFAULT_OPTIONS["fb_power"] = {"interval": 1}
+    DEFAULT_OPTIONS["fb_power"] = {"interval": 10, "post_process": True}
 
 AVAIL_MONITORS = {
     "mpstat": mpstat.MPStat,