docs(changelog): correct arch-diff note — old diff sees the added If, not in-branch changes

The previous wording over-claimed that #328 introduces a change "the old
top-level diff could not see". The old diff DOES report the newly-added
per-layer If phase-split nodes. Reword to the accurate future-proofing
framing: #328 adds the first control-flow subgraph structure; once those If
nodes exist, FUTURE changes inside their branches would be invisible to the
old (subgraph-collapsing) diff, so the recursion is landed now to keep
in-branch changes visible going forward.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: titaiwangms

CHANGELOG.md

@@ -29,15 +29,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - The Architecture-Diff CI tool (`src/mobius/_graph_diff.py`) now recurses into
   `If` / `Loop` / `Scan` `GRAPH`-typed attributes instead of collapsing them to a
-  bare type string, so changes *inside* control-flow subgraphs are visible in the
+  bare type marker, so changes *inside* control-flow subgraphs are visible in the
   diff. A dedicated `subgraph_structure_change` (MODERATE) severity is emitted for
   structural in-branch deltas (node added/removed/rewired, branch added/removed),
-  while pure inner-attribute tweaks stay `changed_attrs` (MINOR). Added here
-  because this PR introduces the static-cache per-layer phase-split
-  `If(Greater(seq_len, 1))` — the first control-flow/subgraph change the old
-  top-level-only diff could not see — so the tool that guards architectural
-  changes must be able to see it. Developer-tooling only; no exported-graph or
-  runtime impact.
+  while pure inner-attribute tweaks stay `changed_attrs` (MINOR). This PR introduces
+  the first control-flow/subgraph *structure* into the exported graphs — the
+  static-cache per-layer phase-split `If(Greater(seq_len, 1))`. The previous
+  top-level diff already reports those newly-added `If` nodes; what it could not do
+  is see *inside* a branch. Once these `If` nodes exist, any future change within
+  their branches would be invisible to the old diff (which collapsed each subgraph
+  to a type marker), so the recursion is added now to keep such in-branch changes
+  visible going forward. Developer-tooling only; no exported-graph or runtime impact.
 
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