Add Dolphin-compatible input expressions and GCPadNew.ini import, DualSense L/R remapping, vibration toggle (#89)

* Add Dolphin-compatible input expressions and GCPadNew.ini import

Rebased onto current main; addresses both CodeRabbit reviews on #89.

- Expression engine matching Dolphin's semantics: doubles rather than
  booleans, 0.5 press threshold, & as min, | as max, and the functions if,
  min, max, clamp, abs, sqrt, pow, sin, cos, tan, deadzone, timer, toggle,
  hold, tap, pulse and smooth. Timing uses a steady clock in seconds, as
  Dolphin does, so a copied expression behaves identically.

- Expressions bind to the GameCube buttons and triggers, combined with the
  existing button mapping rather than replacing it, and are skipped while
  the settings overlay holds input.

- Import reads [GCPadN] from the Dolphin config directory or from
  GCPadNew.ini beside the executable. Stick axes are not expression driven
  and keep their normal mapping.

- Fixes #74: a digital button bound to L or R now reports a fully pulled
  analog trigger, plus a PlayStation preset and a vibration toggle.

Review fixes: config paths round-trip through RuntimeConfigFile::PathToUtf8
and PathFromUtf8 so non-ASCII paths open correctly on Windows, and the
duplicated exists branch is gone; the tap count is clamped before the
unsigned conversion; the expression editor uses resizable storage via
ImGuiInputTextFlags_CallbackResize so a long expression cannot be saved
truncated; clamp bounds are ordered before std::clamp; <cstdlib> is included
for std::strtod; non-finite values are rejected at the evaluator boundary as
well as at the deadzone and timer divisions; and InputBindings::Reload() runs
from InitializeRuntimeSettings rather than the vibration handler.

runtime/tests/test_expr.cpp covers operator precedence, each stateful
function and every case raised in review.

Third review round: smooth() guards NaN as well as infinity so a zero rate
cannot latch a non-finite value in node state; division evaluates both operands
so stateful functions in the left subtree still update when the divisor is zero;
the expression editor clears stale errors when the port changes; and
runtime/tests/test_expr.cpp is registered with CTest as mkw_input_expr_tests,
following the existing test targets.

* Update runtime/src/input_expr.cpp

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

* Update runtime/src/input_expr.cpp

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

* Update runtime/src/input_expr.cpp

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

---------

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

runtime/CMakeLists.txt

@@ -277,6 +277,15 @@ target_link_libraries(mkw_platform_paths_tests PRIVATE mkw_platform)
 target_compile_features(mkw_platform_paths_tests PRIVATE cxx_std_17)
 add_test(NAME mkw_platform_paths_tests COMMAND mkw_platform_paths_tests)
 
+# The input expression engine is self-contained, so it can be exercised without
+# linking the runtime or SDL.
+add_executable(mkw_input_expr_tests
+    "${CMAKE_CURRENT_LIST_DIR}/tests/test_expr.cpp"
+    "${CMAKE_CURRENT_LIST_DIR}/src/input_expr.cpp")
+target_include_directories(mkw_input_expr_tests PRIVATE "${CMAKE_CURRENT_LIST_DIR}/include")
+target_compile_features(mkw_input_expr_tests PRIVATE cxx_std_17)
+add_test(NAME mkw_input_expr_tests COMMAND mkw_input_expr_tests)
+
 # HostContext deliberately keeps the platform-specific context primitive out
 # of fiber_manager.cpp. Exercise the Linux libco handoff directly so future
 # refactors cannot silently remove its headers, implementation, or link edge.