feat(examples): demonstrate competing inventory lifecycles

[yordis]

• Concurrent inventory workflows need a concrete proof that optimistic concurrency serializes decisions without turning event IDs into global uniqueness constraints.
• Multiple reserve and release cycles make distinct operation identity and durable retry boundaries observable after the stream advances.

[cursor]

PR Summary

Low Risk
Changes are limited to an example and integration tests; no production client or runtime behavior is modified.

Overview
Expands the idempotent reservation example from a single-client retry into a two-checkout race on inventory of 1, using expected stream revision so only one reserve wins and the other gets WrongExpectedVersion.

The example now models reserve → release lifecycles with distinct OperationId (event id) and ReservationId, stream folding via read_inventory, and the loser re-reading the stream before appending again. After several successful writes, it replays six stale appends (original revision + same event id) and asserts WriteResult.position matches the first successful write—showing idempotency is tied to revision + id, not a global unique event id.

Adds integration coverage in competing_reservations_are_serialized_and_retryable in idempotency.rs (same flow with fold_inventory and event-id ordering assertions) and registers it in the idempotency test suite.

^{Reviewed by Cursor Bugbot for commit 68f2be3. Bugbot is set up for automated code reviews on this repo. Configure here.}

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📥 Commits

Reviewing files that changed from the base of the PR and between 87784f2 and 68f2be3.

📒 Files selected for processing (2)

• examples/idempotent_reservation.rs
• trogon-eventstore/tests/api/idempotency.rs

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