Postprovision build wait key quoting

Motivation

Clean validation caught scripts/dev/postprovision.sh failing after all three ACR builds completed:

scripts/dev/postprovision.sh: line 157: elb: unbound variable

The build tracker used associative array keys such as elb-terminal without consistently quoting them under set -u.

User-facing change

azd up / postprovision can now wait for dashed image names (elb-api, elb-frontend, elb-terminal) without aborting after successful builds.

API/IaC diff summary

• Quote associative array keys when assigning, reading, and unsetting RUNNING entries in scripts/dev/postprovision.sh.

Validation evidence

• Re-run of scripts/dev/postprovision.sh against rg-elbverify0517 is in progress after the fix.