Move bridge C sources to src/native_bridge/ (consolidation step 1, take 2)

First attempt (commit fb60a8c, reverted by c7d71b0) used filesystem
symlinks in darwin/Classes/ pointing outside the pod's directory tree.
CocoaPods 1.16 silently rejects source_files whose resolved path lies
outside the pod's enclosing dir — pod install crashed at "Adding source
files" with an unhelpful "undefined method `map' for nil" from the
error-reporting fallback.

Replace the symlink approach with committed real-file copies under
serious_python_bridge/darwin/Classes/ that the podspec keeps in sync
with src/native_bridge/ at pod-evaluation time via a small Ruby block:

  __native_bridge = File.expand_path('../../../native_bridge', __dir__)
  if File.directory?(__native_bridge)
    FileUtils.cp ... # refresh Classes/ from canonical source
  end

Monorepo dev: each `pod install` refreshes Classes/ from the canonical
src/native_bridge/. End users pulling the published pod from pub.dev
have only the committed copies; the relative path doesn't exist for
them, so the sync block is a no-op and CocoaPods sees real files inside
the pod dir as expected.

HEADER_SEARCH_PATHS updated to point at "${PODS_TARGET_SRCROOT}/Classes"
rather than the previous "../native".

Other platforms (linux/windows/android) use add_subdirectory("../../native_bridge")
directly — CMake doesn't care about CocoaPods's directory boundary rule.

CI workflow paths (ci.yml Android cross-build + test-bridge-build.yml
Windows-Debug shim compile) and serious_python_bridge/python/setup.py's
NATIVE_ROOT also rewritten to the new path.

Author: FeodorFitsner

.github/workflows/ci.yml

@@ -535,9 +535,9 @@ jobs:
           $CC -shared -fPIC -fvisibility=hidden \
             -DPy_LIMITED_API=0x030c0000 \
             -I"$INCLUDE_DIR" \
-            -I"src/serious_python_bridge/native" \
-            src/serious_python_bridge/native/dart_bridge.c \
-            src/serious_python_bridge/native/dart_api/dart_api_dl.c \
+            -I"src/native_bridge" \
+            src/native_bridge/dart_bridge.c \
+            src/native_bridge/dart_api/dart_api_dl.c \
             "$LIBPYTHON" \
             -Wl,-z,max-page-size=16384 \
             -o "$OUT"

.github/workflows/test-bridge-build.yml

@@ -63,7 +63,7 @@ jobs:
           $vcvars = "$vsPath\VC\Auxiliary\Build\vcvars64.bat"
           Write-Host "vcvars: $vcvars"
 
-          $shimSrc = (Resolve-Path 'src/serious_python_bridge/native/dart_bridge_shim.c').Path
+          $shimSrc = (Resolve-Path 'src/native_bridge/dart_bridge_shim.c').Path
           $includeDir = (Resolve-Path "pywin-$pyver/include").Path
           $libDir = (Resolve-Path "pywin-$pyver/libs").Path
           $outDir = "$PWD\debug_build"
@@ -222,8 +222,8 @@ jobs:
           $CC -shared -fPIC -fvisibility=hidden \
             -DPy_LIMITED_API=0x030c0000 \
             -I"$INCLUDE_DIR" \
-            -I"src/serious_python_bridge/native" \
-            src/serious_python_bridge/native/dart_bridge_shim.c \
+            -I"src/native_bridge" \
+            src/native_bridge/dart_bridge_shim.c \
             "$LIBPYTHON" \
             -ldl \
             -Wl,-z,max-page-size=16384 \