refactor(runtime): drop the unread LaunchContext.data_dir and comment the supervisor rebind

[mfw78]

From the #193 review.

What

• Remove LaunchContext.data_dir. The launcher (AssembledRuntime::launch) never reads it; the local store gets the data directory via BuilderContext.data_dir at component-build time (host/component/builder.rs), so this copy was dead weight. Removed from the struct and all four constructions (the two builder launch paths, bootstrap::run, and a test).
• Comment the let mut supervisor = supervisor rebind so the reason (the dispatch_* calls take &mut self) is not a mystery.

Why top-of-train

LaunchContext and its constructions are edited by several M0 cars (executor threading, WASI-clock override, launch-message wording), so removing the field mid-stack collides with those. Doing it once at the top removes the field from the final shape in a single commit.

Not included

The reviewer notes on the boot-comment direction and the "using ctrl-c" log were already fixed downstream by the drop-CLI-wording and signal-registration cars, so they need no change here.

Testing

cargo clippy -p nexum-runtime -p nexum-cli --all-targets --all-features -- -D warnings clean; builder tests pass.

AI Assistance

Claude Code (Opus 4.8) used for the change and PR body; the train rebasing was driven with jj.

[mfw78]

Superseded by the consolidated follow-up PR #278, which carries this change as a preserved commit. Closing to reduce the outstanding-PR count; the commit history is unchanged.