Filter out SPMI metric diffs smaller than ±0.001% (#125684)

## Description

SPMI metric diff reporting shows noise as changes even when there are no
actual code diffs. The `write_metricdiff_markdown_summary` function used
a bare `!=` equality check, so any tiny numeric variance was reported as
significant.

Inlined an `abs(compute_pct(...)) >= 0.01` threshold check directly in
the list comprehension filter in `superpmi.py` to suppress metric diffs
below ±0.01%. This is consistent with the existing `is_significant_pct`
threshold pattern already used in `write_tpdiff_markdown_summary`.

A dummy comment change was also added to `src/coreclr/jit/compiler.cpp`
to trigger the SPMI pipeline and validate the fix.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>JIT: Metrics show changes even with no-diff
changes</issue_title>
> <issue_description>E.g.
https://dev.azure.com/dnceng-public/public/_build/results?buildId=1339686&view=ms.vss-build-web.run-extensions-tab
has no diffs, yet several metrics show up as changing. We probably
should increase the expected variance of these metrics so that they
don't show up unless they are actually changing.
> 
> cc @EgorBo </issue_description>
> 
> <agent_instructions>Please do not report diffs for SPMI metric diffs
if the difference is smaller than +/- 0.01%</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #125683

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>
Co-authored-by: Egor Bogatov <egorbo@gmail.com>

Author: Copilot

src/coreclr/scripts/superpmi.py

@@ -3564,7 +3564,7 @@ def fmt_val(v):
     metrics_with_diffs = set()
     for metric in all_metrics:
         significant_diffs = [(mch, base, diff) for (mch, _, base, diff) in metric_diffs
-                             if metric in base and base[metric] != diff[metric]]
+                             if metric in base and abs(compute_pct(base[metric], diff[metric])) >= 0.001]
         if not significant_diffs:
             continue