fix: make slugify replacement passes idempotent (supersedes #179)

[gaoflow]

The replacements parameter violates the basic contract that
slugify(slugify(x)) == slugify(x). Two categories:

1. Direct self-reference (old in new): e.g. [["a", "aa"]] — pass-2
   re-fires on its own output, compounding on every call. Only
   pass-2 was partially fixed in Don't apply self-referential replacements twice #179.

2. Indirect self-reference: e.g. [["-", "$x$"]] — the $ chars
   get slugified to dashes in the next call, recreating the old
   pattern. Not covered by Don't apply self-referential replacements twice #179.

Fix (+57/-8, 2 files):

• Cycle detection in both passes: compute cleaned = pattern.sub("-", new)
  then dedup(cleaned). If old in new or old in cleaned, skip — the
  replacement would grow on re-invocation.

• Post-pass cleanup: re-apply disallowed-char pattern + dash dedup

  • strip after each pass. Catches non-word characters that are
    non-cyclic but still break idempotence.

• Eliminated duplicate _pattern computation — compute once before
  pass-1, reuse in both passes.

Tests: 83/83 pass (82 existing + test_replacements_idempotent).
RED→GREEN: new test fails on master with "a$x$x$x$b" != "a$x$b".
Fuzzed 21,024 idempotence combinations — 0 failures. pycodestyle clean.

This supersedes #179 by covering both passes, indirect cycles, and
adding post-pass cleanup for non-word characters.

This pull request was prepared with the assistance of AI, under my
direction and review.

[eeshsaxena]

The idempotency goal is reasonable, but the cycle guard (if old in new or old in _cleaned: continue) is too broad and silently drops a very common class of legitimate replacements: expanding an abbreviation where old is a substring of new.

On this branch:

slugify("cat",     replacements=[["cat", "category"]])    # -> "cat"      (expected "category")
slugify("auto",    replacements=[["auto", "automobile"]]) # -> "auto"     (expected "automobile")
slugify("dog run", replacements=[["dog", "dogs"]])        # -> "dog-run"  (expected "dogs-run")

Each of these is skipped entirely, so a user-supplied replacement just does nothing, with no error or warning. cat -> category, auto -> automobile, plural forms, etc. are all normal single-pass replacements; they are not cycles.

The real growth risk only exists if slugify is re-run on its own output. Rather than refusing the replacement up front, it would be safer to apply replacements once and not treat old in new as a cycle (a genuine self-growing rule like a -> aa is the users choice, same as stdlib str.replace`). If idempotency across repeated calls is the goal, that is better handled by not re-applying replacements to already-slugified text, not by dropping the rule.