spec: ISC_SWEEP gate for the corpus sweep measurement

The per-example timeouts already bound the sweep (16,262 examples,
16k pass, 6 fail — all iso-mal-Mlym-Latn-15919-2001 on the Ruby
compiler; the Interpreter passes the same map, so it is a codegen
divergence, not a corpus gap). The gate keeps CI on the legacy scope
while the conformance tail closes.

Author: ronaldtse

spec/interscript_spec.rb

@@ -2,8 +2,16 @@
 
 cache = {}
 mask = ENV["TRANSLIT_SYSTEM"] || "*"
-# The legacy .imp sweep only (see legacy_maps in spec_helper).
-maps = legacy_maps(select: mask)
+# The ISC corpus sweep runs behind ISC_SWEEP=1: each example is
+# timeout-guarded (at least one map hangs the interpreter) and
+# per-map failures are expected until the ISC-era conformance work
+# completes — the sweep exists to MEASURE that gap, gated so CI stays
+# green while it closes.
+if ENV["ISC_SWEEP"]
+  maps = Interscript.maps(basename: false, select: mask)
+else
+  maps = legacy_maps(select: mask)
+end
 
 # Precache can be used to compare interpreter to compiler performance
 if ENV.include? "PRECACHE"