Add LP-DiD launch carousel (local-projections crossover narrative)

carousel/README.md

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+# Carousel decks
+
+LinkedIn launch carousels for diff-diff estimators. Each deck is a
+`generate_*.py` script that renders its committed PDF; the PDF is a build
+artifact of the script and must never be edited independently.
+
+## Regenerating
+
+```bash
+python carousel/generate_lpdid_carousel.py     # -> diff-diff-lpdid-carousel.pdf
+python carousel/generate_spillover_carousel.py # -> diff-diff-spillover-carousel.pdf
+# ... one generator per deck, same pattern
+```
+
+**Whenever a generator changes, regenerate its PDF in the same commit.** The
+decks carry methodology claims (equivalences, parity tolerances, validation
+scope), so a stale PDF can ship an overclaim that the script no longer makes.
+
+## Dependencies
+
+Carousel generation additionally requires `fpdf2` and `Pillow`, which are
+intentionally NOT part of the library's install or dev extras; `matplotlib`
+is already available through the dev/docs extras but is not a runtime
+install dependency. Python >= 3.9.
+
+```bash
+python -m pip install fpdf2 Pillow
+```
+
+## Claim discipline
+
+Slide copy that states methodology facts (equivalence targets, tolerances,
+validated scope) must match `docs/methodology/REGISTRY.md` and the paper
+review under `docs/methodology/papers/`. Qualifiers are part of the claim --
+e.g. "Cengiz-style" stacking, "BJS-style" imputation (exact only in the
+single-cohort PMD case), "default path" for LPDiD survey support. See the
+"Claim discipline" section in each generator's module docstring for the
+deck-specific rules, and keep footer version labels derived from
+`pyproject.toml` (never hard-coded).