Move design essays to docs/design.md, leave pointers

The three block essays in sras_compute.py — memory budget / row
chunking, angle-alignment coordinate frames, and the sidecar placement
+ schema history — move to docs/design.md (joined by a new section on
the zoom FFT peak search), each replaced by a 2-4 line pointer. The
save_manual_alignment docstring no longer restates the JSON schema its
own eight lines of code construct.

Load-bearing trap notes (normalization=None, subpixel-score mixing,
memmap lazy reads) stay in place. Zero code changes — golden-hash diff
verified empty.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -6,3 +6,4 @@ __pycache__/
 *.sras
 baseline*.txt
 after*.txt
+sras_viewer.egg-info/