docs: add attribute system implementation design

Design doc covering attributes.pm, MODIFY/FETCH callbacks,
variable attribute dispatch, and implementation phases.

Baseline: 62/216 (28.7%) across attrs.t, attrproto.t, attrhand.t, uni/attrs.t.
Target: ~157/216 (73%).

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: fglock