Add introductory lecture slides to methodology vignette

Bring the lecture-slide version of vignettes/methodology.qmd ("Estimating
Incidence Rates from Cross-Sectional Serosurveys") and its figures forward
from the simple-sim branch (PR #393) onto current main, slides only.

Note: the R chunks still call the pre-rename API (load_curve_params ->
load_sr_params, curve_params -> sr_params) and library(serodynamics), so the
deck does not yet render against current main; that update is follow-up work.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-morrison

DESCRIPTION

@@ -1,7 +1,7 @@
 Type: Package
 Package: serocalculator
 Title: Estimating Infection Rates from Serological Data
-Version: 1.4.0.9012
+Version: 1.4.0.9013
 Authors@R: c(
     person("Kristina", "Lai", , "kwlai@ucdavis.edu", role = c("aut", "cre")),
     person("Chris", "Orwa", role = "aut"),

NEWS.md

@@ -1,5 +1,10 @@
 # serocalculator (development version)
 
+## Documentation
+
+* Added introductory lecture slides to the `methodology` vignette
+  ("Estimating Incidence Rates from Cross-Sectional Serosurveys").
+
 ## Internal
 
 * Ported the `@claude` agent and PR-review GitHub Actions workflows (plus Claude/Copilot config: `CLAUDE.md`, `.claude/` settings and slash commands, and path-scoped `.github/instructions/`) from the UCD-SERG `qwt` template, adapted to this package. (#523)