docs(architecture): note sidebar-vs-tier divergence in one sentence

why: The sidebar now uses four navigation buckets (Domain
Packages, UX, Utils, Internal) while the architecture page
continues to describe dependency layers in three tiers.  A reader
looking at both could reasonably wonder whether the discrepancy
is a bug.  Call out the two-axis framing explicitly so the
choice reads as intentional: sidebar = reader-facing grouping;
tier map = dependency ordering.
what:
- Append one sentence after the opening paragraph of
  docs/architecture.md clarifying that the four sidebar buckets
  are orthogonal to the three dependency tiers documented below.
- No other content change; the three-tier structure stays as-is.

Author: tony

docs/architecture.md

@@ -6,6 +6,10 @@ Twelve workspace packages in three tiers.  Lower layers never depend on
 higher ones — domain packages consume shared infrastructure, and the
 presentation layer wires everything together for downstream projects.
 
+The sidebar groups these twelve packages into four navigation buckets
+(Domain Packages, UX, Utils, Internal) — a reader-facing grouping that
+is orthogonal to the dependency-ordered tier map below.
+
 ## Tier 1: Shared infrastructure
 
 The rendering pipeline that all domain packages consume: