Spec-conformance fix: correct stub levels and add BPMN-DI

Three corrections grounded in the UAPF SSOT specification (UAPFormat/
UAPF-specification, specification/01-concepts.md, 04-folder-structure.md,
05-level-composition.md, 10-conformance-checklist.md), which had not been
read in full before the initial workspace build.

1. Level relabel. The FG3 sub-process stubs fg3-2, fg3-3 and fg3-6 had
   been marked level: 4 by template inheritance from fg3-1 at Step 4 of
   the build, despite carrying no BPMN and no resources. Per the spec
   conformance checklist this fails the L4 requirement. The three are
   composition placeholders, which the spec models as L3 (composed
   subprocess / variant). Their uapf.yaml is now level: 3 with
   cornerstones.bpmn: false — conformant: L1-L3 packages MUST NOT
   duplicate L4 content. The three real executables fg3-1, fg3-4 and
   fg3-5 remain L4.

2. BPMN Diagram Interchange. All five .bpmn files in the workspace now
   carry a bpmndi:BPMNDiagram with BPMNShape and BPMNEdge elements
   produced by a swim-lane left-to-right auto-layout, so the diagrams
   preview in bpmn.io, Camunda Modeler and ProcessGit's web view. The
   spec doesn't require DI (its own examples have none) but practical
   reviewability does.

3. Transcoder. tools/register-transcoder gains bpmn_di.py — also runnable
   standalone for retrofitting existing BPMN files. transcode.py now
   imports it and emits DI by default for newly generated skeletons.
   sample-output/3.5.2.skeleton.bpmn and 3.5.3.skeleton.bpmn regenerated
   with DI; the logical-model content is byte-identical to the previous
   commit, only DI is added.

docs/methodology.md updated: adds an explicit Workspace-structure section
grounding L0-L4 in the SSOT spec, a Conformance-correction section
documenting the Step-4 mislabel and its fix, and drops the now-untrue
'no DI' line from limitations.

Validation after the change, full L1-L4 sweep: uapf-cli validate green on
all 10 packages (domains/gramatvediba, fg1-fg6, fg3, fg3-1..fg3-6);
xmllint clean on all 8 .bpmn/.dmn; every .bpmn has BPMNDiagram present.

processes/fg3-6/uapf.yaml

@@ -2,7 +2,7 @@ kind: uapf.package
 id: vk.gramatvediba.fg3-6
 name: "FG3-6 — Kopsavilkuma grāmatošana"
 description: "Summary posting: periodic aggregation and posting of the liability and expense entries arising from the FG3 sub-processes."
-level: 4
+level: 3
 version: 0.1.0
 includes: []
 cornerstones: