fix: add [.spec_tbl_df method to drop spec/problems on subsetting (#569)

[LeonidasZhak]

Closes #569.

What

Add a [.spec_tbl_df S3 method to vroom that drops the spec and problems attributes and removes the spec_tbl_df subclass when a data frame is subset with [.

Why

When vroom creates a data frame, it applies the spec_tbl_df class and attaches spec and problems attributes. Without a [.spec_tbl_df method, these attributes persist after subsetting, which can confuse users and tools that inspect them. readr already has this method; vroom should too.

Changes

• R/utils.R: Add [.spec_tbl_df method (same pattern as as.data.frame.spec_tbl_df, as_tibble.spec_tbl_df, etc.)
• R/zzz.R: Register conditionally via s3_register("base::[", "spec_tbl_df") when readr is not loaded
• tests/testthat/test-vroom.R: Add test verifying that subsetting drops class and attributes

Validation

• devtools::test(filter = "vroom$"): 354 pass, 0 fail
• devtools::test(): 1164 pass, 5 pre-existing failures (locale encoding in test-path.R), 0 new failures
• Verified method works both with and without readr loaded

[LeonidasZhak]

I am withdrawing this PR as part of a broader cleanup of an oversized automated contribution batch. I am sorry for the review noise and the pressure this may have put on maintainers. Going forward I will keep contributions to this project much more limited and higher-signal, ideally only one or two focused PRs at a time. Thank you for maintaining the project.