fix: routeTableStringFormat

[mr-robot-in]

Old behavior (when routeTableStringFormat is false or unset): keeps using ${subnetGroupName}${routeIndex}RouteEntry.
New behavior (when routeTableStringFormat is true): uses ${subnetGroupName}-${routeName-or-sanitized-cidr}-Route, consistent with SubnetStack and honoring route.routeName if provided.

---

Note

Adds routeTableStringFormat to ISubnetsProps and uses it to format subnet route table entry names; updates README and API docs.

• Network construct (src/constructs/network.ts):
  • Adds optional ISubnetsProps.routeTableStringFormat.
  • Uses it to compute route IDs when calling subnet.addRoute, switching from ${subnetGroupName}${routeIndex}RouteEntry to ${subnetGroupName}-${routeName|sanitized-cidr}-Route when enabled, honoring route.routeName.
• Docs:
  • API: documents ISubnetsProps.routeTableStringFormat.
  • README: notes configurable route table entry naming feature.

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