Harmonise condition handling and messaging

[dragosmg]

At present the package uses a mixture of base R (e.g. stop()), {rlang} (abort()) and {cli} (cli_abort()). I suggest we choose one and stick with it. I propose {cli}.

[alanahjonas95]

Task breakdown

Given the size of this change, we'll tackle it in small batches (2-3 files each) so each PR is easy to review. Each batch should:

• Switch stop()/abort() calls to cli::cli_abort() / cli::cli_warn() / cli::cli_inform()
• Revisit message wording for clarity (first line = the problem, i/x bullets for detail)
• Use cli inline formatting ({.arg}, {.val}, {.fn}, {.cls}) and bulleted/numbered lists where helpful
• Use the shared check_*() helpers where applicable (see Batch 1)
• Keep lines under 80 characters
• Add/extend expect_snapshot() tests to cover the messaging

Batch 1 - Shared infrastructure (do first, merge before other batches)

• Agree on message style conventions
• Add/extend rlang-style check_*() helpers for common type/arg checks in tfrmt_checks.R / struct_utils.R
• Add an initial expect_snapshot() test as the pattern for later batches

Batch 2

• tfrmt_checks.R
• struct_utils.R

Batch 3

• tfrmt.R
• tfrmt_layer.R

Batch 4

• body_plan.R
• col_plan.R
• col_style_plan.R

Batch 5

• row_group_plan.R
• page_plan.R
• footnote_plan.R

Batch 6

• frmt_plans.R
• tfrmt_n_pct.R
• tfrmt_sigdig.R

Batch 7

• apply_tfrmt.R
• apply_col_style_plan.R
• apply_page_plan.R

Batch 8

• apply_row_grp_plan.R
• apply_table_frmt_plan.R
• apply_footnote_meta.R
• apply_frmt_methods.R

Batch 9

• big_n.R
• JSON.R
• extract_data.R

Batch 10

• print_to_gt.R
• print_to_ggplot.R
• prep_card.R

Batch 11

• shuffle_card.R
• display_insights.R

[alanahjonas95]

For message style conventions, I'd propose:

Aspect	Convention	Example
First line	State the problem, imperative, no period	"{.arg x} must be a single string"
Use {.arg name}	For argument names (rendered with backticks)	{.arg column}, not "column"
Use {.val value}	For actual values (syntax highlighted)	{.val c("a", "b")}
Use {.fn func}	For function names	{.fn tfrmt()}, {.fn footnote_plan()}
Use {.cls class}	For class names	{.cls tfrmt}, {.cls big_n_structure}
Pluralize with {?s}	Use cli::pluralize(), not manual ifelse	"value{?s}" expands to "value" or "values" automatically
Bullets	Use c("Main problem", "i" = "info", "x" = "specific issue", ...)	Multiple "x" for each invalid item, "i" for guidance
Call context	Always include call = caller_env()	Points error to user's call, not internal code
Class naming	Use class = "tfrmt_error_descriptive_name" (no leading _)	For programmatic error handling by users
Line length	Keep wrapped lines <80 chars	Makes diffs readable, respects terminal widths