docs(architecture): clarify LangGraph state-mutation timeline in §6.2.1

Reflow the §6.2.1 diagram so node order mirrors the runtime topology
declared in src/support_bot/application/answering/graph.py:
START → retrieve → guard → _guard_edge → {generate | refuse} → END.

Changes:
- Group nodes under "Stage 1..4b" subgraphs aligned with execution order.
- Add inter-stage runtime arrows labeled t1/t2/t3 between RN→GN→E→DEC
  and the two final branches (t3a policy: not refuse → generate,
  t3b policy: refuse → refuse).
- Reaffirm that Stages 4a/4b are mutually exclusive at runtime but
  shown together because they share the same mutate contract.
- Keep the existing edge-type legend (==read==>, ==write==>,
  ==append==>) and the per-node writes/append contract table
  unchanged.

Strict mkdocs build clean (3 pre-existing TOC anchor INFO warnings
unrelated to this change).

Author: bruj0