Restore bare `fn` package compatibility and add tidyverse/testthat smoke CI

[isomorphisms]

Problem

IR currently reserves bare fn as a function-constructor spelling. The IR README already notes the consequence: old R code using fn as a variable or argument must be changed.

That is a package-ecosystem compatibility break, not just a cosmetic source change. rlang uses fn in live source code; for example R/trace.R contains a formal named fn (call_trace_context <- function(call, fn)) and assigns to fn inside the function. rlang is foundational for testthat, tibble, dplyr, and the tidyverse.

This matters immediately for translating r-koans into IR: the lessons can be rewritten, but the standard teaching/testing stack should not have to be forked merely to get past the parser.

Preferred invariant

Unmodified R packages should continue to parse and load under IR unless IR intentionally changes their semantics. New IR syntax should be additive where possible.

The smallest compatibility-preserving option appears to be to stop reserving ASCII fn and keep λ / ƒ as the distinctive function-constructor spellings. A more contextual parser is also possible, but the acceptance criterion should be package compatibility rather than a particular implementation.

Add a package smoke gate

Build IR, then install/load unmodified current releases of at least:

• rlang
• testthat
• tibble
• dplyr

Optionally add the tidyverse meta-package once the smaller gate is reliable.

Then run an upstream testthat test containing IR syntax, for example:

library(testthat)

test_that("IR spellings work inside upstream testthat", {
  answer ← 8 ÷ 2
  expect_true(answer ≟ 4)
})

Also parse a representative dependency source file that contains a bare fn identifier so this exact regression cannot come back silently.

Why not rewrite testthat first?

Keep upstream testthat unchanged as a compatibility oracle. If we immediately translate/fork the test framework itself, the tests can accidentally hide a language-compatibility defect. IR-native test code can sit on top of upstream testthat once the package stack loads cleanly.

Acceptance criteria

• unmodified rlang installs and loads under IR
• unmodified testthat installs and loads under IR
• unmodified tibble installs and loads under IR
• unmodified dplyr installs and loads under IR
• a testthat test written with ←, ÷, and ≟ passes
• CI contains a regression fixture with a bare identifier/formal named fn
• the final README states clearly whether ASCII fn remains reserved

Related motivation: translating isomorphisms/r-koans into IR.