Framework: an exact selector for a fixed-seed unit (Rerun lines are coarse -t filters today) #78

Follow-up from #58 (Windows CI). A failing fixed-seed unit prints a Rerun command that is a coarse -t filter: a sibling test whose name matches the same anchored pattern also runs. Example: a unit named fixture Q"1 replay and a sibling fixture Q21 replay share a filter that a regular-expression wildcard in the name can match.

The framework has no selector for a fixed-seed unit. ORACLE_UNIT is read only for a random unit during a path replay (oracle-property.ts assertReplayUnit, oracle-environment.ts), so a fixed unit cannot be selected exactly by an environment variable.

Proposed: a framework selector for a fixed-seed unit (for example ORACLE_FIXED_UNIT, read the same way as ORACLE_UNIT), with a Rerun line that uses it, and an oracle RED/GREEN for it (framework changes are oracle-first, P9). Acceptance: a Rerun line for Q"1 runs that unit only, and not Q21.

Recorded as a Limit on #58 until this lands.