Fix package-lock-violation crash when typing " in REPL

in-pathname-context-p used (intern ... :cl) to check if a function was
a pathname function.  When the enclosing form started with a dotted
package name like zr.strings:parse-f64, find-enclosing-function
returned "zr" (stopping at the dot), and (intern "ZR" :cl) triggered
an SBCL package-lock-violation on the locked COMMON-LISP package.

Fix: use find-symbol instead of intern — we only need to look up the
symbol, not create it.

Also: widen form-complete-p error handler from reader-error to error,
and fix find-string-start's (incf i) inside (loop for i) bug.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>