Treat a single NA as NULL for export_tfl_page annotations

[billdenney]

What

A single NA supplied for an annotation or presence-toggle argument in export_tfl_page() now behaves exactly like NULL (the "absent" state), so data-driven page construction (one row per page) can pass NA for an unsupplied value instead of rendering the literal text "NA" or erroring.

Covered arguments:

Argument(s)	NA behavior
caption, footnote, header_left/center/right, footer_left/center/right	section element absent (identical to NULL)
page_i	no "Page <i>: " error prefix
header_rule, footer_rule	no rule (FALSE)

How

Implemented once via a shared predicate .is_single_na() in R/normalize.R (length-1 atomic NA of any type), consumed by normalize_text(), normalize_rule(), and a page_i coercion in export_tfl_page().

Because annotations flow through normalize_text() after the x[[key]] override merge, a per-page x$caption = NA now also blanks an otherwise-global caption. Only a single NA is absent — a longer vector with NA among real values (c("a", NA, "b")) is still collapsed with "\n" as before, so multi-line content is never silently dropped.

Considered but deliberately left to fail loudly

• padding, margins, min_content_height — unit objects; NA is malformed, caught by checkmate.
• caption_just, footnote_just, content_just, overflow_action — enumerated match.arg settings with concrete defaults; an NA signals a construction bug and should error.
• gp, preview, newpage, x — not presence toggles.

Rationale recorded as D-52 in design/DECISIONS.md.

Tests

• test-normalize.R: .is_single_na() truth table, normalize_text(NA) across atomic types, the multi-element-NA boundary, normalize_rule(NA).
• test-export_tfl_page.R: a grid.grab() display-list label sweep asserting no literal "NA" reaches the page for each of the 8 annotations, all-NA renders identically to none-supplied, the x-override blanking case, page_i = NA, and header_rule/footer_rule = NA (previously threw). These fail against the pre-fix code.

Full test suite passes. Docs regenerated via roxygen; CLAUDE.md and all design/ docs updated.

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