During a GIL-heavy from_pretrained the engine's event loop is blocked, so its
/internal/engine-state poll times out and the engine looked "down" with an empty
task list — the real loading task never reached the front. Parse load progress
from the engine's log stream (which the front already pumps) into Engine.loading
and surface it as a synthetic 'loading' task (with live step/total) in
_merge_engine_tasks, even when the primary engine is the blocked one. Cleared on
"Model loaded successfully" or the next successful poll.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>