fix(make): export EXAMPLE variable for sub-make invocations

[Alex-ai-future]

Summary

make run-example kv_cache_index silently ran offline instead.

Root Cause

run-example calls sub-makes via $(MAKE), but Make doesn't pass internal variables to sub-makes by default.

The sub-make re-evaluates EXAMPLE from MAKECMDGOALS. Its command is make run-example-only (one word), so $(word 2,$(MAKECMDGOALS)) is empty and it falls back to the default offline.

Fix

Added export EXAMPLE so the value is inherited by sub-makes as an environment variable.

Why CI didn't catch this

CI calls make run-example-only <name> directly in hack/verify-examples.sh, bypassing the run-example wrapper entirely. run-example-only receives the full argument list, so MAKECMDGOALS is correct. Only the run-example wrapper (used by local developers) triggers this bug.

Impact

Command	Before	After
make run-example kv_cache_index	silently ran offline	correctly runs kv_cache_index
make run-example online	silently ran offline	correctly runs online
make run-example (no arg)	ran offline	no change

[Alex-ai-future]

After testing, this is an actual bug that appears in Mac and Linux at the same time.

[github-actions]

This PR is marked as stale after 21d of inactivity. After an additional 14d of inactivity (7d to become rotten, then 7d more), it will be closed. To prevent this PR from being closed, add a comment or remove the lifecycle/stale label.

[yankay]

Thanks @Alex-ai-future
/lgtm