Offline mode

Set JRT_OFFLINE=1 and jrt refuses every network operation, while installed runtimes and tools keep working.

PowerShell

$env:JRT_OFFLINE = '1'
jrt run black --version        # works: already materialized

What is blocked

These fail immediately with a clear error instead of hanging on a dead connection:

jrt tool add and jrt tool update (metadata resolution)
jrt runtime install and jrt backend bootstrap
package-manager engine lookups that are not cached yet
RubyGem and Maven artifact downloads, and jrt compat update <https-url>

What launched tools are told

Every process jrt starts receives the offline switches of its own ecosystem, so a missing piece fails closed rather than silently resolving something different:

Variable	Effect
`MISE_OFFLINE=1`	mise never downloads
`GOPROXY=off`, `GOSUMDB=off`	Go never fetches modules
`CARGO_NET_OFFLINE=true`	Cargo stays offline
`NPM_CONFIG_OFFLINE=true`	npm uses its cache only
`PIP_NO_INDEX=1`	pip never contacts an index