Add zvol existence check and creation for iSCSI extents

client.py:
- check_iscsi_zvols(): queries pool.dataset.query for VOLUME type,
  returns list of missing zvol names
- create_zvol(): creates a single zvol via pool.dataset.create
- create_missing_zvols(): opens a fresh connection and creates a
  batch of zvols from a {name: volsize_bytes} dict

summary.py:
- Add zvols_to_check and missing_zvols list fields
- Report shows a WARNING block listing missing zvols when present

migrate.py:
- _migrate_iscsi_extents() populates summary.zvols_to_check with
  the dataset name for each DISK-type extent during dry run

cli.py:
- Add _parse_size() to parse human-friendly size strings
  (100G, 500GiB, 1T, etc.) to bytes
- run() calls check_iscsi_zvols() during dry run and stores results
  in summary.missing_zvols
- Wizard prompts for size and creates missing zvols after the dry
  run report, before asking the user to confirm the live run

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

truenas_migrate/summary.py

@@ -52,6 +52,10 @@ class Summary:
     paths_to_create:  list[str] = field(default_factory=list)
     missing_datasets: list[str] = field(default_factory=list)
 
+    # Populated during iSCSI dry-run zvol safety checks
+    zvols_to_check:  list[str] = field(default_factory=list)
+    missing_zvols:   list[str] = field(default_factory=list)
+
     @property
     def _has_iscsi(self) -> bool:
         return (self.iscsi_extents_found + self.iscsi_initiators_found +
@@ -147,5 +151,16 @@ class Summary:
                 "  These paths must exist before shares can be created.\n"
                 "  Use interactive mode or answer 'y' at the dataset prompt to create them."
             )
+        if self.missing_zvols:
+            lines.append(
+                f"\n  {_bold_yellow('WARNING:')} "
+                f"{len(self.missing_zvols)} zvol(s) do not exist on the destination:"
+            )
+            for z in self.missing_zvols:
+                lines.append(f"    {_yellow('•')} {z}")
+            lines.append(
+                "  These zvols must exist before iSCSI extents can be created.\n"
+                "  Use interactive mode to be prompted for size and auto-create them."
+            )
         lines.append("")
         return "\n".join(lines)