iter 8: prompt rule 29 for CMOS 8.159 headline-case prepositions

Real-draft testing on the HML bibliography surfaced inconsistent
preposition capitalization in headline-style titles: the formatter
was sometimes lowercasing "among", "beyond", "within", "into", etc.
and sometimes capitalizing them, violating CMOS 8.159. The behavior
was nondeterministic across similar entries — smoking-gun evidence
of GPT-5 judgment drift on a rule the existing prompt only
implicitly covered via rule 6's "headline-style capitalization".

Rule 29 makes the carve-out explicit and instructs the model to
apply CMOS 8.159 independently of the source's casing, with:

- a substantial but illustrative preposition list including "as"
  (which CMOS 8.159 calls out as always lowercased)
- an explicit scope restriction to PREPOSITIONS, with subordinating
  conjunctions (If, That, Because, Although, Unless, etc.)
  capitalized
- a CMOS 8.161 carve-out for hyphenated compounds: the first
  element is always capitalized (so "In-School" stays, not
  "in-School")
- a first/last-word exception covering "last word of the main
  title immediately before a subtitle colon", which the model
  was treating as mid-title

Verified on the 134-entry HML draft: ~17 entries now produce more
CMOS-correct forms, and two consecutive re-runs show no new rule-
29-related regressions. Observed GPT-5 drift on orthogonal axes
(multi-author inversion, inner-quote style, periodical
italicization) washed out across re-runs, consistent with the
~5% single-run variance documented in project memory.

Rule count: 28 -> 29. No linter version bump required.

src/cmos/formatter.py

@@ -235,6 +235,79 @@ Rules you must follow (CMOS 18th edition specifically):
     capitalization. When in doubt about whether a lowercased form is
     intentional, err on the side of preserving the source's form.
 
+29. LOWERCASE PREPOSITIONS IN TITLES — CMOS 8.159. CMOS headline-style
+    capitalization lowercases ALL prepositions regardless of length,
+    UNLESS the preposition is the first word or last word of the title
+    or subtitle, or is used adverbially or adjectivally. This is a
+    distinctive CMOS rule and differs from AP style and many other
+    house styles, which capitalize prepositions of 5+ letters. Rule 6
+    ("headline-style capitalization") does NOT override this — this
+    rule is the carve-out.
+
+    Common prepositions that MUST be lowercased mid-title (illustrative,
+    not exhaustive):
+    about, above, across, after, against, along, among, around, as, at,
+    before, behind, below, between, beyond, by, during, for, from, in,
+    inside, into, of, on, onto, out, over, through, throughout, to,
+    toward, under, until, up, upon, with, within, without.
+
+    The word "as" is specifically listed in CMOS 8.159 as always
+    lowercased in titles, whether functioning as a preposition or a
+    conjunction ("Information Literacy as Sociocultural Practice", NOT
+    "Literacy As Sociocultural").
+
+    The infinitive marker "to" is also lowercased ("How to Read a Book",
+    not "How To Read a Book").
+
+    SCOPE — PREPOSITIONS ONLY. This rule covers prepositions. Do NOT
+    apply it to SUBORDINATING CONJUNCTIONS such as "If", "That",
+    "Because", "Although", "Though", "Whether", "Unless" — these are
+    CAPITALIZED in headline-style titles. CMOS 8.159 lowercases only
+    COORDINATING conjunctions (and, but, for, or, nor, so, yet), not
+    subordinating conjunctions. Do not over-apply rule 29 and lowercase
+    words that are not prepositions.
+
+    HYPHENATED COMPOUNDS — CMOS 8.161. In a hyphenated compound within
+    a title, the FIRST element is ALWAYS capitalized, regardless of
+    part of speech, even when that first element would be lowercased
+    as a standalone preposition under this rule. Do NOT apply rule 29's
+    lowercasing to the first element of a hyphenated compound.
+    Examples:
+    - "In-School Adolescents" (NOT "in-School")
+    - "On-Campus Housing" (NOT "on-Campus")
+    - "Post-Trump Academic Librarianship" (first element capitalized)
+    - "Anti-Racist Initiatives" (first element capitalized)
+    The second element of a hyphenated compound follows CMOS 8.161
+    separately: usually capitalized if it is a noun, verb, or adjective,
+    but lowercased if it is a short article, preposition, or
+    coordinating conjunction.
+
+    CRITICAL: apply this rule INDEPENDENTLY of the source's casing. The
+    source may have the preposition capitalized or lowercased; the
+    correct output is always lowercase (unless an exception applies).
+    Do not preserve a source's wrong capitalization, and do not flip a
+    source that already has it right.
+
+    Correct examples:
+    - "Information among Librarians" (not "Information Among Librarians")
+    - "Libraries Moving beyond Performative Politics" (not "Moving Beyond")
+    - "Experience within Animal Environments" (not "Experience Within")
+    - "An Exploration into Archival Materials" (not "Exploration Into")
+    - "Testing Intentions among Young Men" (not "Intentions Among")
+    - "Bathrooms Inclusive If the Stacks Exclude" (NOT "if" — subord. conj.)
+    - "A Study That Changed Everything" (NOT "that" — subord. conj.)
+
+    First-word / last-word exceptions (CAPITALIZE in these positions):
+    - "Beyond the Pale: A Study of X" ("Beyond" is the first word)
+    - "The Decade After" ("After" is the last word of the title)
+    - "A Report on Knowledge" ("on" is mid-title, stays lowercase)
+    - "In Academia and Beyond: Resisting Stigma" ("Beyond" is the last
+      word of the MAIN title immediately before a subtitle colon — it
+      is capitalized, not lowercased. The main title and the subtitle
+      are treated as SEPARATE units for the first/last-word exception.
+      Similarly, the first word of a subtitle is always capitalized
+      regardless of part of speech.)
+
 Output ONLY the single reformatted entry. No preamble, no explanation.
 """