fix(ck-engine): parse lexical queries leniently instead of erroring on tantivy syntax (#166)

ck is grep-shaped, so lexical (--lex) queries arrive as arbitrary strings — from
shell one-liners, scripts, and AI agents — that were never meant to be tantivy
query syntax. QueryParser::parse_query rejects the whole search when the input
contains anything it can't fully interpret: an unbalanced quote, a stray
`field:` colon, a bare AND/OR, a leading wildcard. The terms that could still
match are discarded along with the syntax error, and callers are forced to
sanitize queries before handing them over.

Switch lexical_search to QueryParser::parse_query_lenient: the interpretable
terms are kept and the un-parseable fragments are dropped, with the parser's
errors logged at debug level rather than propagated. A query that already parses
cleanly yields the same query object, so its results and scores are unchanged
(covered by an invariance test comparing strict and lenient parses); a query
where every fragment errors degrades to normal empty results instead of a hard
failure.