Support native-aware wrapper install/update flows

[realhaley]

Closes jbangdev#2458

What this changes

This PR adds a first pass at native-aware wrapper install/update behavior.

Wrapper behavior

• when JBANG_USE_NATIVE=true, wrapper scripts now:
  • look for a native binary first
  • try downloading an OS/arch-specific native bundle if the binary is missing
  • fall back to the generic bundle if no native bundle is available
  • fall back to jbang.jar execution if the installed bundle does not contain a native binary

Native-aware update/install path

• jbang version --update and install flow now:
  • use the same native-aware bundle resolution when JBANG_USE_NATIVE=true
  • copy native binaries when present
  • write jbang.bin(.exe).new alongside jbang.jar.new for existing installs
• wrapper startup now promotes pending native .new files into place

Configurable base URL

• added JBANG_DOWNLOAD_BASEURL to override the release base URL
• this is used for:
  • wrapper bootstrap downloads
  • Java-side install/update downloads
  • version check (version.txt)
• intended to make local testing possible against a fake release tree or local HTTP server

Build / CI / release bits

• adds native bundle tasks:
  • nativeDistZip
  • nativeDistTar
• adds CI steps to build and upload native bundles
• tag/release workflow downloads those bundles before release
• JReleaser config updated toward including native bundle artifacts

Notes

Current public releases do not yet publish the new OS/arch-specific native bundles, so wrapper fallback to generic bundles is still expected until release artifacts are wired all the way through.

This PR also adds:

• docs/native-wrapper-testing.md

with a local file:// and local HTTP testing flow for the new behavior.

Tested

• ./gradlew -q compileJava
• bash -n src/main/scripts/jbang
• local smoke test using JBANG_DOWNLOAD_BASEURL=file://...
• local native-requested fallback flow when no native bundle exists