#683: use >= 0 for sign selection in badge.xsl to avoid displaying -0

[morphqdd]

Closes #683.

xsl/badge.xsl's sign-selection branch used strict $avg > 0, so when $avg is exactly "0.0"
(zero facts, or a rounded-to-zero average), the condition is false and the otherwise-branch emits
-, while abs($avg) emits 0 — the badge shows -0.

Widened the sign check to $avg >= 0, matching the existing correct pattern already used for
color selection in vitals.xsl ($i >= 0).

Added test_shows_plus_zero_when_average_is_exactly_zero to test/pages/test_badge.rb, which
transforms a minimal <fb/> (zero facts → avg = 0.0) through the real badge.xsl via Saxon and
asserts the rendered text is +0, not -0.

Verified directly with Saxon-HE 9.8.0-5 (downloaded jar): confirmed the badge renders -0 without
the fix and +0 with the fix, for the same zero-average input.

Note: I could not run this new test locally through Minitest — qbash (used by the existing
xslt() test helper and my new test) hardcodes /bin/bash, which doesn't exist on this NixOS
sandbox — but this is an environment artifact unrelated to the change (confirmed via a direct
Saxon invocation outside the test harness, and the real CI test job runs on Ubuntu where
/bin/bash exists).

[morphqdd]

@yegor256, please take a look when you have a moment.