roster.client_version empty for 31/46 agents - version drift is unmeasurable fleet-wide

roster.client_version is empty for 31 of 46 agents seen in the last 24h, including pm-llmmsgsrv-cc, coder-llmmsgsrv-cc, nw-whey-cc and every scrp-* emitter. Only 15 rows carry a version: 2.44.80 (7), 2.17.14 (4), 2.44.71 (2), 2.18.14 (2). Mechanism: llmmsg-srv-mcp.mjs:154 stamps x-llmmsg-client = llmmsg-srv-mcp/SHIM_VERSION on outbound requests, SHIM_VERSION read once at shim process start from the package.json SIBLING to the shim file (lines 24-28). Something is not populating or not persisting that header for most agents - unknown whether the header is absent, the hub is not recording it on the paths those agents use, or the column is only written on certain endpoints. CONSEQUENCE, which is why this is worth filing: any statement of the form 'roster shows agent X on version Y' is only answerable for a minority of the fleet, and an EMPTY value is indistinguishable from 'current' when reading the table - a fleet-wide version-drift sweep would silently pass 31 agents it never measured. Found while driving the whey 2.18.14 drift (bin-whey-cc + proxy-mba-w-cc). PM ruling 2026-08-15: file it, do not chase now.