fix(codegen): arity-aware method namespace fallback + drop 4 fixed sweep entries

[zaidejjo]

Fixes all 5 sweep unexpected results blocking the v0.2.0 release CI:

1. regexp_test native C compile error: untyped-receiver method dispatch picked 3-value str.find for 2-value ph.find(x) (latent since feat(stdlib): bounded search (find_in/rfind_in/count_in) #286 made find 3-arg; match-bound receiver type never reaches the callgraph). All three fallback sites now prefer the arity-compatible candidate (receiver+args), falling back to old behavior when arity is unknown. Only ever changes selection away from a guaranteed compile error. Also adds regexp to the last-resort namespace list.

2. Removes selective_import, multi_selective, symbol_alias, wildcard_import from known_native_failures (sweep reports FIXED; verified VM-identical native output locally).