Use count-based progress for Docker image pulls

Refactor Docker image pull progress to use a simpler count-based approach
where each layer contributes equally (100% / total_layers) regardless of
size. This replaces the previous size-weighted calculation that was
susceptible to progress regression.

The core issue was that Docker rate-limits concurrent downloads (~3 at a
time) and reports layer sizes only when downloading starts. With size-
weighted progress, large layers appearing late would cause progress to
drop dramatically (e.g., 59% -> 29%) as the total size increased.

The new approach:
- Each layer contributes equally to overall progress
- Per-layer progress: 70% download weight, 30% extraction weight
- Progress only starts after first "Downloading" event (when layer
  count is known)
- Always caps at 99% - job completion handles final 100%

This simplifies the code by moving progress tracking to a dedicated
module (pull_progress.py) and removing complex size-based scaling logic
that tried to account for unknown layer sizes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/docker/test_interface.py

@@ -709,11 +709,18 @@ async def test_install_progress_handles_layers_skipping_download(
         await install_task
         await event.wait()
 
-        # First update from layer download should have rather low progress ((260937/25445459) / 2 ~ 0.5%)
-        assert install_job_snapshots[0]["progress"] < 1
+        # With the new progress calculation approach:
+        # - Progress is weighted by layer size
+        # - Small layers that skip downloading get minimal size (1 byte)
+        # - Progress should increase monotonically
+        assert len(install_job_snapshots) > 0
 
-        # Total 8 events should lead to a progress update on the install job
-        assert len(install_job_snapshots) == 8
+        # Verify progress is monotonically increasing (or stable)
+        for i in range(1, len(install_job_snapshots)):
+            assert (
+                install_job_snapshots[i]["progress"]
+                >= install_job_snapshots[i - 1]["progress"]
+            )
 
         # Job should complete successfully
         assert job.done is True
@@ -844,24 +851,24 @@ async def test_install_progress_containerd_snapshot(
         }
 
     assert [c.args[0] for c in ha_ws_client.async_send_command.call_args_list] == [
-        # During downloading we get continuous progress updates from download status
+        # Count-based progress: 2 layers, each = 50%. Download = 0-35%, Extract = 35-50%
         job_event(0),
+        job_event(1.7),
         job_event(3.4),
-        job_event(8.5),
+        job_event(8.4),
         job_event(10.2),
-        job_event(15.3),
-        job_event(18.8),
-        job_event(29.0),
-        job_event(35.8),
-        job_event(42.6),
-        job_event(49.5),
-        job_event(56.0),
-        job_event(62.8),
-        # Downloading phase is considered 70% of total. After we only get one update
-        # per image downloaded when extraction is finished. It uses the total size
-        # received during downloading to determine percent complete then.
+        job_event(15.2),
+        job_event(18.7),
+        job_event(28.8),
+        job_event(35.7),
+        job_event(42.4),
+        job_event(49.3),
+        job_event(55.8),
+        job_event(62.7),
+        # Downloading phase is considered 70% of layer's progress.
+        # After download complete, extraction takes remaining 30% per layer.
         job_event(70.0),
-        job_event(84.8),
+        job_event(85.0),
         job_event(100),
         job_event(100, True),
     ]