Add changelog fragment and document IO-benchmarking conventions

Adds the performance-changelog entry and captures in AGENTS.md the two
conventions this effort leaned on: how to write changelog fragments
(type by intent — performance work is newfeature; one-sentence entries
ending with a contributor name link anchored in names.inc) and the
interleaved-A/B benchmarking discipline used to validate these speedups.

Author: bruAristimunha

AGENTS.md

@@ -151,3 +151,15 @@ changelog entry instead of a plain name link.
   # (BSD-compatible).
   ```
   If the license of a snippet cannot be determined, do not adapt it.
+
+- Benchmarking performance changes: only interleaved A/B runs against a
+  pristine snapshot built from the exact upstream base are trustworthy;
+  whole-suite back-to-back runs drift ±20–100 %. Verify which installed mne a
+  benchmark actually imports (`print(mne.__file__)`) before trusting numbers,
+  and keep fixture data out of commits.
+- Changelog fragments (`doc/changes/dev/<PR#>.<type>.rst`): pick `<type>` by
+  intent — performance improvements are `newfeature`, not `bugfix`. Keep the
+  entry to one short sentence ending with the contributor name link, e.g.
+  "Speed up X by optimizing Y, by `Jane Doe`_", and make sure the name anchors
+  in `doc/changes/names.inc` (add it if missing). Read an existing fragment
+  or two before writing yours.

doc/changes/dev/14212.newfeature.rst

@@ -0,0 +1 @@
+Speed up reading by optimizing small read paths, by `Bruno Aristimunha`_