Fix R CMD check WARNINGs blocking CRAN submission

[WestonVoglesonger]

R CMD check --as-cran on main currently reports 4 WARNINGs (consistent across ubuntu/macos/windows, all R versions), which block CRAN submission (#31). The cran-comments.md template still claims "0 warnings", so it's out of date.

Warnings

1. LazyData — LazyData DB of 13.6 MB without LazyDataCompression set. Fix: add LazyDataCompression: xz to DESCRIPTION.
2. namespace references in data files — data/fit_sim.rda: lubridate. Root cause (verified by walking the object): fit_sim does not contain a lubridate-classed value. The reference lives in stanfit@.MISC$stan_fit_instance (the compiled-model Rcpp instance), whose captured environment references the lubridate namespace. It was captured because data-raw/DATASET.R runs library(tidyverse) (attaching lubridate) when it calls sampling(). imuGAP's own code never uses lubridate (not in R/, NAMESPACE, or DESCRIPTION).
3. Rd \usage — Undocumented arguments in Rd file 'create_target.Rd': 'mode'. The mode arg (added in export create_target and add a new mode #63) lacks a roxygen @param mode.
4. install — Warning: namespace 'lubridate' is not available and has been replaced. Same root cause as attempt the NC application of the pipeline (JHU has a good draft) #2: loading fit_sim resolves the captured lubridate reference, which warns when lubridate isn't installed.

Fix (no re-sampling required)

Verified that stanfit@.MISC is a disposable cache: predict() reconstructs what it needs from @stanmodel via gqs(). Blanking .MISC and re-saving keeps predict working (4000-row output) and removes the lubridate reference entirely — the posterior draws are untouched, so no downstream test values shift.

• Scrub the shipped fixture in place: load fit_sim, fit$stanfit@.MISC <- new.env(), re-save via usethis::use_data(fit_sim, compress = "xz", overwrite = TRUE). Fixes attempt the NC application of the pipeline (JHU has a good draft) #2 and add outbreak results, validate swapping between state-views, and update with NC wording feedback on the dashboard #4 without re-running Stan.
• Harden data-raw/DATASET.R: replace library(tidyverse) with only the packages actually used (splines, data.table, EnvStats) so a future regeneration doesn't reintroduce the capture.
• LazyDataCompression: xz in DESCRIPTION → Incorporate the finalized process model into the package #1.
• @param mode on create_target, re-document → Clean up the pipeline, making clear the bespoke vs generic boundaries #3.

(Rejected alternative: adding lubridate to Imports would silence #2/#4 but declares an unused dependency and leaves the leak in the fixture.)

Also seen (NOTE, non-blocking)

• S3 generic/method consistency NOTE — likely summary.imugap_predict (change predict returns to be draws-compatible #70). Worth cleaning while here.

Acceptance

• R CMD check --as-cran = 0 errors, 0 warnings on all CI platforms
• fit_sim still drives predict examples/tests after the scrub
• cran-comments.md updated with the real note count

Refs #31