fix(features): inherit mv hashing salt in change requests and scheduled commits

[Zaimwa9]

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Changes

Contributes to #7913

Builds on #7914. Two recreation paths bypass clone() and still re-bucketed multivariate variants:

• v1 change request / scheduled change commits: drafts are new rows, so every commit re-dealt variants. ChangeRequestCommitService now carries the superseded live state's salt onto drafts at commit time.
• Recreating a segment override in a draft version: this tripped the new guard with an unactionable 400. The version feature state serializer now inherits the salt from the previous version's matching override.

Also: succession helpers on FeatureState (used by clone()), a features.feature_state.mv_variants_rebucketed warning event for unknown v1 recreation paths (added to the events catalogue), and clone() copies multivariate values in id order so variant order is deterministic.

Alternatives considered

• Implicit inheritance (BEFORE_CREATE hook): covers unknown paths too, but heals them silently — a flow bypassing clone() would never surface and be fixed. Rejected in favour of declared, reviewable call sites plus the guard tripwire (raise under v2, warning event under v1).
• Routing all recreation through clone() (copy the old state, apply the request on top): the request already defines the entire new state, so the clone would contribute nothing but the salt — while starting from old values instead of defaults silently changes what partial requests return, and the rewired create path would touch shared serialisers far beyond the two affected flows. Possible later refactor.

How did you test this code?

New unit tests: variant assignment stable across a change request commit (fails without the fix), override delete + recreate in a draft inherits the salt via the API, warning event fires only for unknown v1 paths, lineage lookup helper.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
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Additional details and impacted files

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##             fix/7913-mv-hashing-salt    #7963   +/-   ##
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