fix: prevent second batch store reset wiping postage snapshot on --resync

[martinconic]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
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Description

Fixes #5495: a node started with --resync while the postage snapshot is active shuts down during sync with:

get: get batch <id>: storage: not found

Root cause

The batch store is reset twice on startup in this configuration:

1. snapshotBatchSvc.Start() resets the store (resync) and rebuilds it from the snapshot, then sets postageSyncStart = maxBlockHeight.
2. The live batchSvc was constructed with the same o.Resync flag, so its Start() reset the store again, discarding the snapshot that was just loaded.

Live sync then started from the snapshot block height against an empty store, so the first top-up/dilute event referencing a batch created before the snapshot height failed with storage: not found, and the node shut down.

This matches the report exactly: two "resync requested, resetting batch store" log lines, and the fact that skip-postage-snapshot works around it (only one reset, and sync replays from contract genesis where every create precedes its top-up).

Note: this is independent of #5343/#5482. The flow originates in #5094 and is still present on master; the reporter's build (2.8.0-41d6efc6) predates the #5343 revert, but reverting #5343 does not address this.

Fix

The two reset sites are not redundant — one rebuilds from the snapshot, the other is the fallback rebuild for the no-snapshot / snapshot-failed / non-mainnet cases — they were just both armed at once.

Construct the live batch service after the snapshot path has run and pass resync only when the snapshot did not rebuild the store:

batchSvc, err = batchservice.New(..., o.Resync && !snapshotLoaded)

The reset logic stays in its single existing place (batchservice.Start()), exactly one service is armed to reset, and there is no runtime toggle. The store is still reset exactly once in every other case (snapshot skipped, failed, not applicable, or a non-resync start).

Testing

• TestResyncControlsReset in batchservice asserts both directions of the contract node.go relies on:
  • resync=true ⇒ store reset once
  • resync=false ⇒ store not reset (the post-snapshot live-service case)
• go test -race ./pkg/postage/batchservice/... passes
• go build ./pkg/node/... passes; go vet, gofumpt, and golangci-lint clean

Open API Spec Version Changes (if applicable)

None.

Related Issue

Closes #5495

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