VIOLET OA: add venue OB provider seam

Co-authored-by: Codex <codex@openai.com>

prod/docs/VIOLET_SPEC__MULTI_EXCHANGE_OB_SEAM.md

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+# VIOLET Multi-Exchange OB Seam
+
+Date: 2026-06-16
+
+## Scope
+
+This note describes the new read-only OB provider seam added on the VIOLET side:
+
+- `prod/clean_arch/violet/venue_ob_provider.py`
+- `prod/clean_arch/violet/test_violet_venue_ob_provider.py`
+
+The seam exists so VIOLET can later consume a venue-specific OB feed without
+changing BLUE or wiring any live exchange connection into the current shadow
+path.
+
+## What the seam does
+
+- normalizes venue ticks into BLUE-shaped `OBSnapshot` records
+- keeps the provider read-only and in-memory
+- validates poison values at ingress with V-TYPES
+- supports either an injected callable tick source or a preloaded buffer
+- exposes the exact `OBProvider` interface that `OBFeatureEngine` expects
+
+## What it does not do
+
+- no live BingX connection
+- no Hazelcast writes
+- no BLUE code changes
+- no live wiring into `live_blue_source.py` or `shadow_live_factors.py`
+
+## Future adapter shape
+
+A venue-specific live adapter can later be layered on top of this seam by
+feeding `VenueOBTick` records into `VioletVenueOBProvider.refresh()` from any
+normalized feed source. The only contract is the canonical `OBSnapshot` shape
+that `OBFeatureEngine` already consumes.
+
+## Validation
+
+The companion test file checks:
+
+- `VioletVenueOBProvider` implements the `OBProvider` surface
+- malformed values are rejected at construction
+- `OBFeatureEngine(provider)` can run `step_live()` and read back finite OB
+  features
+- the callable-source path loads as expected
+