docs: ARCHITECTURE_COMPARISON.md as living source file

Per J's request: move the parallel-runtime comparison from
reports/cutover/ (where it lived as cutover-prep evidence) into
docs/ as the source-of-truth file. J will keep updating it as
fixes ship on either side.

Restructured for living-document use:
- Status header (last refresh date, owner, update triggers)
- 'How to update this doc' section with explicit dos and don'ts
- Decisions tracker at top — actioned items with commit refs
  + open backlog with LOC estimates
- Each comparison section now has 'Last verified' columns where
  numbers are time-sensitive
- Change log section at bottom for one-line entries on every
  meaningful refresh

The original at reports/cutover/architecture_comparison.md gains
a 'THIS IS A SNAPSHOT' header pointing at the docs/ source. Kept
as historical record but no longer the place to update.

Sister pointer file in /home/profit/lakehouse/docs/ARCHITECTURE_COMPARISON.md
so the doc is reachable from either repo side. That file explicitly
says the source lives in golangLAKEHOUSE and warns against
authoritative content in the pointer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

reports/cutover/architecture_comparison.md

@@ -1,4 +1,9 @@
-# Lakehouse: Rust vs Go architecture comparison
+# Lakehouse: Rust vs Go architecture comparison (snapshot)
+
+> **THIS IS A SNAPSHOT — NOT THE SOURCE OF TRUTH.**
+> The living document is at **`docs/ARCHITECTURE_COMPARISON.md`**.
+> Update there; this file is a frozen historical record.
+> Snapshot date: 2026-05-01.
 
 Produced 2026-05-01 to inform the keep/maintain decision and surface
 abstractions that should be addressed regardless of which side is the