Replace deprecated xfun attribute getters with xfun::attr2()

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• ## Description

  • This PR removes deprecated xfun attribute access patterns by replacing read-time attr() usage with xfun::attr2() across affected package code.
  • It also updates related docs/version metadata and adds a guard test to prevent deprecated xfun attribute helpers from reappearing.

• ## Changes

  • Attribute getter migration
    • Replaced attribute reads in affected R functions (print.*, summary.*, autoplot.*, compare_seroincidence, est_seroincidence_by, utils, strata, etc.) with xfun::attr2().
    • Kept attribute assignment (attr(x, "...") <- value) unchanged.
  • Regression coverage
    • Added tests/testthat/test-xfun-deprecations.R to assert:
      • no xfun::atrr() usage
      • no xfun::attr() usage
      • no non-assignment attr() getter usage in R/
  • Docs and metadata
    • Updated roxygen-generated docs (man/set_id_var.Rd, man/summary.seroincidence.by.Rd).
    • Added NEWS entry under development ## Internal.
    • Incremented dev version in DESCRIPTION to 1.4.1.9033.

• ## Example

# before
start <- object |> attr("lambda_start")

# after
start <- object |> xfun::attr2("lambda_start")

• Fixes Update use of deprecated xfun functions #643

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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