dispatcher: retire Tier-4 legacy resolve_slot + add public SlotManager.state(); document the two routing axes

[thinmintdev]

What to build

The Dispatcher's Tier-4 legacy fallback proxy.resolve_slot (path/name heuristics) is reached at exactly one site and is superseded by Tiers 1-3. Retire it: delete Tier-4, raise NoRouteFound when Tiers 1-3 miss (confirm no model bindings rely on the legacy path first). Also add a public SlotManager.state(name) -> SlotState and switch the Dispatcher off the private _current_state() leak. Add a short ARCHITECTURE note documenting the two orthogonal routing axes (Dispatcher = model-id→URL transport; route_for_request = capability→slot selection) so this isn't misread as redundant again.

Do NOT touch omni_router / route_for_request — they stay (verified load-bearing for omni's tool selection + dispatch).

Pointers: dispatcher/proxy.py:58, dispatcher/router.py:549,669,704. Audit Q2.1.

Acceptance criteria

• Tier-4 resolve_slot removed; Tiers 1-3 raise NoRouteFound on miss
• Public SlotManager.state() added; Dispatcher no longer calls _current_state()
• ARCHITECTURE documents the two routing axes
• omni_router untouched; suite green

Blocked by

None - can start immediately

[thinmintdev]

Heads-up: PR #649 (attempted retirement) shows Tier-4 resolve_slot is still the live capability-model→slot resolver for audio + image dispatch — removing it makes /v1/audio/* and /v1/images/* return dispatch.no_route (test_v1_audio/test_v1_images go red on a previously-green main). So #624 is not pure cleanup: it needs audio/image resolution migrated off Tier-4 into tiers 1–3 first. Re-scoping required before this can land.

[thinmintdev]

Reframing: closing the cleanup framing. PR #649 proved Tier-4 resolve_slot is load-bearing for audio/image dispatch, so retiring it is a routing-migration feature, not tidy-up: the capability-model→slot resolution Tier-4 performs must move into tiers 1-3 first, with /v1/audio/* + /v1/images/* integration coverage kept green. Deferring to a deliberate design pass (draft PR #649 holds the exploratory diff + findings). Not an AFK-sweep item.