Enhancement: Match fuzzy terms in place inside the parsed query (#14157)

* Feature: match fuzzy terms in place inside the parsed query

Fuzzy matching was a separate clause OR'd in above the query: a flat bag
of the query's words, re-parsed through tantivy's own parser, blended
beside the exact clause. Nothing around a term reached it, so a fielded
term fuzzed across every default field, a filter did not constrain it, and
an exclusion had to be hoisted back over the whole blend to stop the
clause re-admitting what the query had just excluded.

Widen each leaf where it sits instead, through emit()'s rewrite_leaf hook,
so fielding, negation, AND, REQUIRE and positive filters constrain the
fuzzy match exactly as they constrain the exact one. Each of a leaf's
words becomes a Fuzzy leaf on the leaf's own field, boosted to 0.1, beside
the leaf and any CJK alternative it already had.

* Hello?

docs/configuration.md

@@ -1279,6 +1279,8 @@ Tantivy stemmer equivalent, stemming is disabled.
 matching. Fuzzy results rank below exact matches. A value of `0.5` is a reasonable
 starting point. Leave unset to disable fuzzy matching entirely.
 
+    Words of a single character are not fuzzy-matched, since a single-character approximate match would match nearly every term in the index.
+
     Defaults to unset (disabled).
 
 #### [`PAPERLESS_SANITY_TASK_CRON=<cron expression>`](#PAPERLESS_SANITY_TASK_CRON) {#PAPERLESS_SANITY_TASK_CRON}