fix(extraction): count all file-level-tracked langs (incl .properties) as indexed (#544)

Completes #357. The no-symbol file-level class is yaml/twig/properties, but the
count fix only covered yaml/twig — so a .properties-only project still printed
"No files found to index" even though the files were stored. Introduce a single
isFileLevelOnlyLanguage predicate (the canonical set behind the tree-sitter
no-symbol branch, xml excluded since its MyBatis extractor emits a file node)
and use it at both count sites and the extraction dispatch so the list can't
drift. Adds .properties regression coverage for indexAll() and indexFiles().

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>