fix(report): use the last CCN average in multi-file lizard XML reports

lizard --xml writes a running cumulative <average label="CCN"> after each
source file's items (all as flat siblings under one <measure> element, since
the format has no per-file nesting). parseLizard's .find() picked the first
one, i.e. the first file's own average, silently under/over-reporting
complexity for any real multi-file project.

Found while scaffolding example-rust: a 4-function first file (all CCN 1)
reported project complexity as 1 instead of the true 2.03 average across all
5 files / 34 functions. Confirms example-typescript's live badge is affected
too.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: tbjers