build(scripts): add per-artifact build scripts staging into dist/

One script per main artifact, all staging into dist/ (overridable via
--dist or VOICECAT_DIST_DIR):

- build-server.sh        server-release preset -> dist/server/
- build-windows-client.sh  windows-client DLL + dotnet publish -> dist/windows-client/
- build-macos-client.sh    XCFramework + xcodebuild VoiceCatMac.app -> dist/macos-client/
- build-libs.sh           libvoicecat per platform -> dist/lib/{macos,ios*,windows}/
                         + dist/lib/VoiceCatCore.xcframework/
- build-all.sh            orchestrator (host-applicable artifacts only)
- common.sh               shared helpers: dist/VCPKG_ROOT resolution, platform
                         guards, logging, --help from header comments

Host-platform guards prevent running the wrong script on the wrong OS.
build-libs.sh and build-macos-client.sh drive the existing validated
clients/apple/scripts/build-xcframework.sh rather than duplicating it.
dist/ added to .gitignore.

.gitignore

@@ -1,6 +1,9 @@
 # Build output
 /build/
 /out/
+
+# Staged distribution artifacts (scripts/build-*.sh output)
+/dist/
 *.o
 *.obj
 *.a