Fix _TOTAL_BAR regex pattern matching

[Hultwl]

Problem
The launcher briefly prints an invalid download percentage before
settling into a normal 0-100% bar, then launches fine (#223).

Root cause
_TOTAL_BAR matched on the prefix of a progress bar's label
(^(initializing|downloading)), not the whole label. The real
aggregate bar's label is exactly "Downloading", but a per-file or
per-segment bar can be captioned "Downloading " — same prefix.
That segment's own tiny, near-complete total briefly gets read as the
whole package's total, so the percentage is computed against the
wrong denominator for one frame, until the next line (the real
aggregate bar) corrects it.

The Fix:
Anchor _TOTAL_BAR to the full label instead of a prefix match:

_TOTAL_BAR = re.compile(r"^(initializing|downloading)\s*$", re.I)

Testing

• pytest tests/test_xodus.py — 83 passed
• Added test_a_segment_bar_labelled_downloading_is_not_the_total_bar,
  which feeds _consume a "Downloading segment_0000.msixvc" line and
  asserts it's ignored, then confirms the real aggregate bar still
  drives progress correctly

Closes #223