feat: add manual release build scripts for macOS and Windows

CI does not have macOS or Windows runners, so provide scripts to
build and optionally upload release archives locally.

- scripts/build-release-macos.sh  — builds x86_64 and aarch64-apple-darwin
- scripts/build-release-windows.ps1 — builds x86_64-pc-windows-msvc
- scripts/.env.example             — template for Gitea credentials

Both scripts read GITEA_TOKEN, GITEA_SERVER_URL, and GITEA_REPOSITORY
from the environment or from scripts/.env (which is gitignored).
Upload to Gitea is skipped when GITEA_TOKEN is not set.

Use curl.exe for multipart upload in the PowerShell script to support
Windows PowerShell 5.1 (Invoke-RestMethod -Form requires PS 6.1+).

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

scripts/.env.example

@@ -0,0 +1,7 @@
+# Copy this file to .env and fill in your values.
+# .env is loaded automatically by the build-release scripts.
+# Environment variables already set in your shell take precedence.
+
+GITEA_SERVER_URL=https://git.example.com
+GITEA_REPOSITORY=owner/mdrs-client-rust
+GITEA_TOKEN=your_token_here