feat: enumerate complete raw maintenance populations

[zzylol]

Why

Group-aware maintenance must enumerate every durable physical population before choosing inputs. Filtering out an unbound historical SID could produce a partial outer summary.

What

Add a private complete raw population inventory over the existing SID metadata and durable parts. Keep the current singleton scheduler gate.

How

Hold the admission and instance snapshot, require finite raw closure, and union durable nonremoved and live SIDs. Every SID must be current, writable, raw, completed, and represented by durable coordinates. Return the existing SID/group/window map.

Before this PR

The coordinate helper rejected multiple SIDs and could only serve the singleton consumer.

After this PR

A future group-aware consumer can enumerate two distinct instance groups across two SIDs. A missing live binding for either retained durable SID rejects the inventory before any target mutation.

Verification

• Four actual sketchlib refactor(legacy_expr): step β — thread Schema through legacy planning stack #139 index-maintenance tests passed in 6.18 s.
• Tests cover two SIDs, two groups within one SID, preclosure rejection, and missing-binding rejection with unchanged part and metadata counts.
• Independent scoped source review passed.
• Process/performance evidence: not applicable to this private inventory layer.

Limits

This is an inventory snapshot, not a publication capability. Consumers must still acquire frozen input proofs and revalidate lifetimes transactionally. Multi-group scheduling, versioned routing, continuous watermarks, and sliding execution remain separate work.

Stacked on #668.