tex.ch: fix hyphenation exception slot and memory leak (S941)

[dangerzig]

Summary

The \hyphenation command is intended to maintain a set of exception words, but re-hyphenating a word unconditionally increments hyph_count and displaces the old entry via Robin Hood hashing rather than replacing it in place. This wastes hash slots and leaks both the old string (from make_string) and the old token list.

The existing web2c workaround in tex.ch decrements hyph_count and flushes the duplicate string, but deliberately skips flush_list(hyph_list[h]) with the comment "it interferes with trip.log."

Proposed fix

Add flush_list(hyph_list[h]) to free the old token list, and remove the comment.

{repeat hyphenation exception; flushing old data}
flush_string; s:=hyph_word[h]; {avoid |slow_make_string|!}
flush_list(hyph_list[h]); {free old hyphen position list}
decr(hyph_count);
goto found;

Trip test impact

This reduces string pool memory by ~5 words at every Memory usage report in trip.log and trip.fot. tripin.log and trip.typ are identical.

Test file

With the default hyph_size = 307:

\newcount\n \n=0
\loop \advance\n by 1
  \hyphenation{test-ing}  % same word, 308 times
\ifnum\n<308 \repeat
\bye

On an unmodified tex.web build, this triggers "TeX capacity exceeded, sorry [exception dictionary=307]" on the 308th insertion, even though only one unique word was defined.

Context

This was reported to Professor Knuth by letter (March 2026) and to tex-k@tug.org in the thread "Three potential bugs in tex.web (Version 3.141592653)". The full changefile patch covering this and 7 overflow fixes was sent to the list for review.

The complete diff (tex-ch-overflow-patches.diff) and cover document (tex-ch-overflow-report.pdf) are attached to the companion overflow issue.