A release/optimized `jolt build` is a closed world: every app def is final, so
an app->app call can bind to the def's Scheme binding directly instead of going
through (jolt-invoke (var-deref ns name)).
The emitter gains a direct-link mode (off for the seed mint, runtime -e/repl, and
dev builds). With it on, a top-level app def also emits a binding jv$<ns>$<name>
that def-var! aliases; an app->app call or value-ref to a name already emitted in
the unit lowers to that binding, skipping both the var-table lookup and the
generic IFn dispatch. ^:dynamic/^:redef defs and nested defs (a defonce's inner
def) opt out and stay indirect. Off direct-link mode, emit-top-form is exactly
emit, so the seed and runtime eval are byte-unchanged (selfhost holds).
build.ss turns it on for release + optimized; the defined-set accumulates across
the dependency-ordered namespaces so a dep's defs are linkable by the time the
entry that calls them is emitted. App->core calls stay indirect for now (core is
the baked seed); that's a later stage.
~1.74x on a hot cross-namespace call loop (26.5s -> 15.2s).
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