feat(model): FlowGraph + LayoutStore foundation for WYSIWYG editor

First piece of the v0.2.0 rewrite. The graph view, text view,
and run view will all share a single source of truth: the
YAML text. This commit introduces the parsing layer + the
sidecar that keeps layout metadata OUT of the YAML.

FlowGraph (lib/src/model/flow_graph.dart):
 - Parses + emits the F-Delta-I flow YAML shape (inputs +
   steps + outputs + leading comment block).
 - Recognises both inputs shorthand (`name: text`) and the
   expanded form (`name: { type: text, default: ..., hint: ... }`).
 - Detects edges by scanning step.with + outputs for
   $source.field references; understands both the short
   $x.y form (canonical) and the long ${{ x.y }} form (legacy).
 - Flags system.approval@ steps via FlowStep.isApproval so
   the graph can paint them with the human-gate styling.
 - Immutable update helpers (withStepUpdated, withStepAdded,
   withStepRemoved) used by the property panel + add-step
   button.
 - tryParse returns null on YAML syntax errors so the text
   tab can keep the last valid graph while the operator
   types.

LayoutStore + FlowLayout (lib/src/model/layout_store.dart):
 - Per-flow JSON sidecar at
   ~/.fai/data/flows/.layout/<name>.json. Atomic write via
   tmp + rename so a crash mid-save leaves the previous
   copy intact.
 - The YAML stays byte-identical to what `fai run` consumes
   — positions never bleed into the flow file.

AutoLayout (lib/src/model/auto_layout.dart):
 - Deterministic topological column layout for flows that
   have never been opened in the graph view before.
 - Steps with no $step refs sit in column 0; steps whose
   refs all point at column-0 steps sit in column 1; etc.
 - Cycles + dangling refs collapse to column 0 so nothing
   is ever invisible.

Tests (test/flow_graph_test.dart): 7 cases cover canonical
parsing, edge detection across both ref forms, approval-step
flagging, leading-comment round-trip, structural round-trip,
parser robustness on broken YAML, immutability invariant.
All pass.

Version 0.1.4 -> 0.2.0 (minor bump — significant new module
graph; public FlowEditorPage constructor API unchanged).

Signed-off-by: flemming-it <sf@flemming.it>

pubspec.yaml

@@ -1,6 +1,6 @@
 name: fai_studio_flow_editor
 description: Swappable inline YAML editor for F∆I Studio flows.
-version: 0.1.4
+version: 0.2.0
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor
 
@@ -16,6 +16,10 @@ dependencies:
   # WebView-free and ships in the same binary.
   flutter_code_editor: ^0.3.5
   highlight: ^0.7.0
+  # YAML parsing for the graph view's model layer. We
+  # round-trip text <-> graph via this; the graph never
+  # mutates the YAML directly.
+  yaml: ^3.1.2
 
 dev_dependencies:
   flutter_test: