feat(flowchart): alignment + distribution tools (C15)

8 new toolbar buttons (Align L/R/T/B, Center H/V, Distribute H/V)
plus a Select All button. Each operation takes the current
selection and applies a pure-function transform to the (x, y)
positions, then commits each result via store.moveNode().

Architecture:

  - src/flowchart/flowchart-align.js: pure module with 8 helpers
    + nodeWidth / nodeHeight utility. UMD wrapper for browser
    global + CommonJS, same pattern as the other pure modules.
  - src/renderer/flowchart-bundle.js: pure module inlined so the
    standalone window doesn't need a separate bundle build; the
    inlined functions are exposed as window.FlowchartAlign for
    jsdom tests.
  - Bundle's _selectedNodeIds Set tracks the alignment selection.
    Empty by default; populated by Select All or by future
    canvas-side shift+click (item 1: multi-select).
  - Each align button no-ops with a status hint if fewer than 2
    nodes are selected. Distribute needs 3+.
  - Each moveNode call creates its own undo snapshot, which is
    fine for typical 2-10 node selections and lets the user undo
    a misalignment one node at a time.

Tests: 14 new (1376 total) covering all 8 functions plus
immutability, no-op for insufficient nodes, and unsorted-input
sorting for distribute.

Amit Haridas

src/flowchart-generator.html

@@ -395,6 +395,74 @@
               Reset All
             </button>
           </div>
+          <h3 class="fc-section-title">Align (multi-select)</h3>
+          <div class="fc-toolbar-row">
+            <button
+              class="fc-btn"
+              id="fc-btn-select-all"
+              title="Select every node so the alignment buttons work"
+            >
+              Select All
+            </button>
+          </div>
+          <div class="fc-toolbar-row">
+            <button
+              class="fc-btn"
+              id="fc-btn-align-left"
+              title="Align selected nodes to the left edge"
+            >
+              Align Left
+            </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-align-right"
+              title="Align selected nodes to the right edge"
+            >
+              Align Right
+            </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-align-top"
+              title="Align selected nodes to the top edge"
+            >
+              Align Top
+            </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-align-bottom"
+              title="Align selected nodes to the bottom edge"
+            >
+              Align Bottom
+            </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-align-center-h"
+              title="Align selected nodes to a common horizontal center"
+            >
+              Center H
+            </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-align-center-v"
+              title="Align selected nodes to a common vertical center"
+            >
+              Center V
+            </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-distribute-h"
+              title="Distribute selected nodes evenly between leftmost and rightmost"
+            >
+              Distribute H
+            </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-distribute-v"
+              title="Distribute selected nodes evenly between topmost and bottommost"
+            >
+              Distribute V
+            </button>
+          </div>
           <h3 class="fc-section-title">Export Image</h3>
           <div class="fc-toolbar-row">
             <button class="fc-btn" id="fc-btn-export-svg" title="Export the canvas as a .svg file">