[C15] Kotlin: emit _ for source underscore locals under K2

A source underscore variable (an unused `_` in a `catch (_: E)` clause or a
`val _ = ...` discard) is named differently by the two frontends:

- K1 keeps the source spelling `_`.
- K2 assigns the synthetic `SpecialNames.UNDERSCORE_FOR_UNUSED_VAR`, which
  renders as `<unused var>`.

Decision (D15): adopt the K1 behaviour. `_` is the actual source token, so it
is the more intuitive and source-faithful name; it also keeps the local
variable name consistent with the corresponding value-parameter case, where a
prior fix already normalises the underscore setter parameter to `_`.

`extractVariableExpr` now maps a variable whose IR name is
`SpecialNames.UNDERSCORE_FOR_UNUSED_VAR` to `_` before writing `localvars`.
The check is on the special name rather than a raw string so it is robust
across compiler versions, and it only fires for the frontend-synthesised
unused-variable name, leaving all other locals untouched.

Full dual-suite relearn: all 3333 tests pass. The only changed expected row is
in query-tests/UnderscoreIdentifier, where the K2 catch parameter row converges
from `Exception <unused var>` to `Exception _`, matching K1. The remaining
divergence in that file (the destructuring container `<destruct>` vs
`tmp0_container`) is a separate naming/location issue tracked under C14.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

java/kotlin-extractor/src/main/kotlin/KotlinFileExtractor.kt

@@ -51,6 +51,7 @@ import org.jetbrains.kotlin.load.java.structure.JavaTypeParameter
 import org.jetbrains.kotlin.load.java.structure.JavaTypeParameterListOwner
 import org.jetbrains.kotlin.load.java.structure.impl.classFiles.BinaryJavaClass
 import org.jetbrains.kotlin.name.FqName
+import org.jetbrains.kotlin.name.SpecialNames
 import org.jetbrains.kotlin.types.Variance
 import org.jetbrains.kotlin.util.OperatorNameConventions
 import org.jetbrains.kotlin.psi.KtClass
@@ -3489,7 +3490,13 @@ open class KotlinFileExtractor(
             val exprId = tw.getFreshIdLabel<DbLocalvariabledeclexpr>()
             val locId = getPsiBasedLocation(v) ?: tw.getLocation(getVariableLocationProvider(v))
             val type = useType(v.type)
-            tw.writeLocalvars(varId, v.name.asString(), type.javaResult.id, exprId)
+            // K2 names a source `_` (unused) local/catch variable with the synthetic
+            // SpecialNames.UNDERSCORE_FOR_UNUSED_VAR (`<unused var>`), whereas K1 keeps the
+            // source spelling `_`. Emit `_` so both frontends produce the same, source-faithful
+            // name (D15).
+            val varName =
+                if (v.name == SpecialNames.UNDERSCORE_FOR_UNUSED_VAR) "_" else v.name.asString()
+            tw.writeLocalvars(varId, varName, type.javaResult.id, exprId)
             tw.writeLocalvarsKotlinType(varId, type.kotlinResult.id)
             tw.writeHasLocation(varId, locId)
             tw.writeExprs_localvariabledeclexpr(exprId, type.javaResult.id, parent, idx)

java/ql/test-kotlin2/query-tests/UnderscoreIdentifier/test.expected

@@ -4,4 +4,4 @@
 | Test.kt:6:9:6:26 | Pair<Integer,Integer> p |
 | Test.kt:7:9:7:26 | Pair<Integer,Integer> <destruct> |
 | Test.kt:7:14:7:18 | int first |
-| Test.kt:8:14:8:25 | Exception <unused var> |
+| Test.kt:8:14:8:25 | Exception _ |