[isomorphisms]

Stop reserving ASCII fn; it now follows the ordinary SYMBOL path again, while λ and ƒ remain function-constructor spellings.

The compatibility smoke:

• exercises a representative function(call, fn) { fn <- ... } fixture;
• verifies both λ and ƒ still construct functions;
• installs unmodified current CRAN rlang from source under IR and loads it;
• runs IR syntax (←, ÷, ≟) independently of packages whose source intentionally relies on standard-R = assignment.

The broader testthat / tibble / dplyr smoke from issue #12 is not claimed here. Its transitive vctrs dependency uses bare = assignment in package source, while IR deliberately gives = equality semantics. That is a separate compatibility boundary from the fn lexer regression. The previous CI also exposed an unrelated missing libuv development header through fs.

The README states that fn is an ordinary identifier and that fn(...) is an ordinary call, not constructor syntax.

This is stacked on #5 because the smoke test uses the preferred ≟ spelling introduced there. It removes the parser blocker for dilapidated-shed/r-koans#2.

Addresses the bare-fn portion of #12; the broader package-stack question remains open.