Backend: Own ZoneKind / zone geometry at one seam

[arniber21]

Parent

#201

Context

Identical three-variant zone enums and near-identical zone structs:

• backend::ZoneKind + NeutralAtomZone (target.rs)
• ZoneKindDescriptor (descriptor.rs)
• quon_na::ZoneKind + ZoneSpec (zoned.rs)
• quonc::na_target::map_zone_kind — rename match only

What to build

Single ZoneKind / zone geometry type owned in backend (or a tiny shared module both depend on). quon_na consumes it. map_zone_kind becomes unnecessary (type alias or deleted).

Soft ADR-0009 note: hoisting the enum (shared vocabulary) is not the “awkward unused Option fields on Fixed” problem that ADR forbids — call this out in the PR.

Acceptance criteria

• One ZoneKind type in the workspace for NA zones
• JSON descriptor → domain → planner uses that type without a match remap
• na_target shrinks to movement/placer params mapping only (or disappears into pipeline)
• Existing NA tests / targets JSON still load
• PR notes ADR-0009 compatibility

Blocked by

None - can start immediately. Align with #211 if zone types move during dual-IR cleanup.

Related

#201, #211, ADR-0009