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>
