Adds 3 cases for the opt-path checker (set-check-mode! -> run-inference ->
take-diags!): diagnostics drain once, the buffer empties on re-drain, and
check-mode off produces none. This is the public checker entry the other 23
cases didn't touch. Runtime-only.
The corpus/unit gates compile through run-passes' const-fold-only branch, so
the type-inference walk runs only under jolt build --opt — buildsmoke hit one
trivial app and checked stdout. run-infer.ss drives the pass directly: analyze
a source string, then call check-form / infer-body / the set-*! registries and
assert diagnostic counts and collected calls/escapes. Wired into make ci.
Gives the inference pass real behavior coverage so refactoring its internal
state is gate-validatable. jolt-ogib.10 groundwork.
