M9-S7: fix CatalogListingView cross-queue create race

[erikshafer]

What

Fixes the CatalogListingView last-writer-wins race between the Listings read-model handlers on different RabbitMQ queues. Two handlers could both LoadAsync the same catalog row as null and both Store a fresh document; the second commit silently overwrote the first, losing selling fields (Title, SellerId, StartingBid). Observed live on 2026-06-13.

OQ-1 spike (the gate) — reshaped the fix

The slice was gated on whether the planned UseNumericRevisions(true) would make the losing Store throw. A diagnostic spike against live Postgres said no:

Strategy	Loser throws?	Final Title	mt_version
Store vs Store (current)	no exception	'' (lost)	2
Insert vs Insert (fix)	DocumentAlreadyExistsException	'SELLING-RECORD' (kept)	1

UseNumericRevisions bumps mt_version but a plain Store still commits and last-writer-wins — enforcement needs a revision-checked write path, which sagas get from Wolverine's saga-persistence frame but plain read-model handlers do not (the same lesson AuctionClosingSaga records from M8-S3c). So the mechanism is the document primary key, not the version column.

Fix

• session.Insert on the create branch of all twelve CatalogListingView write methods across five handlers — via a shared IDocumentSession.InsertOrStore helper for the ?? new sites, and direct Insert in the two explicit-null branches.
• New ListingsConcurrencyRetryPolicies : IWolverineExtension retries on DocumentAlreadyExistsException (mirrors AuctionsConcurrencyRetryPolicies). The retried handler re-loads the committed row and merges via the existing load-and-preserve Store path, so both field sets survive.
• The spike's IRevisioned/UseNumericRevisions edits were reverted — inert and misleading for a plain-Store path. CatalogListingView.cs is byte-identical to baseline.

Scope boundary

Closes the insert-insert create race (the observed bug). The residual update-update race (two handlers updating an existing row) is deferred as OQ-3 — never the observed failure, and closing it would mean UpdateRevision across four handler files. Tracked alongside the analogous Operations BC read-model pattern.

Tests

• 2 new bidirectional regression tests (collision throws + retry merges, both commit orderings).
• No contracts/events/spec changes. No existing test modified.
• 328 backend tests green (+2). Build clean (1 pre-existing unrelated AppHost MessagePack advisory).

Prompt + retrospective included: docs/prompts/implementations/M9-S7-… and docs/retrospectives/M9-S7-….