feat(r): box::use/library imports + module\$fn calls (#218, #219)

R had no import extraction at all (not in the import dispatch), so library(),
require(), source() AND box::use() all produced zero IMPORTS edges; and
module\$fn() calls were dropped because the call-callee resolver did not handle
R extract_operator function nodes.

#218: add parse_r_imports — recursively scan for call nodes and emit IMPORTS for
library/require/requireNamespace/loadNamespace/source (first arg) and for each
box::use(mod[syms], pkg/path[syms], ...) argument (module path = the spec text
before the [symbols] list). AST node types confirmed via a tree-sitter-R probe.

#219: handle extract_operator (R \$) as a call function node in
extract_callee_from_fields → emit "module.fn" so module\$fn() yields a CALLS
edge like other member calls.

Tests cover box::use + library + source imports and the \$-call; r_collect_imports
added to the recursion whitelist.