Fully remove gt dependency after the as_gt() deprecation period

Background

PR #629 replaced the heavy gt dependency with the lightweight lt package. To avoid breaking user code that customizes output with {gt} functions, as_gt() was given a gentle deprecation rather than being removed outright:

• as_gt() still returns a gt_tbl object, but emits .Deprecated() pointing to lt::lt().
• gt was moved from Imports to Suggests; as_gt() checks for it at call time and errors with an install hint if it is missing.
• lt() S3 methods are the recommended replacement and are available now.

Task (future release)

After at least one release of the gentle deprecation, fully remove {gt}:

• Remove R/as_gt.R (the as_gt() generic and its S3 methods) and man/as_gt.Rd.
• Drop the S3method(as_gt, *) entries and export(as_gt) from NAMESPACE (regenerate via roxygen).
• Remove gt from Suggests in DESCRIPTION.
• Remove the assert_gt_installed() helper.
• Remove as_gt from _pkgdown.yml and any lingering references in docs/vignettes/tests.
• Add a NEWS entry noting the removal.

xref #629

Start by reading R/as_gt.R and searching the repository for as_gt, gt, and assert_gt_installed, including NAMESPACE, DESCRIPTION, _pkgdown.yml, docs, vignettes, and tests. Regenerate NAMESPACE via roxygen and run the package checks. Done means the deprecated API, gt dependency, helper, references, and documentation are removed, with a NEWS entry recording the change.