feat: add insertMany to repository API (Batch accounts 2/5)

[gantunesr]

Stack part 2 of 5 split from #601. Adds repository-level batch lookup and insert APIs, nullable account tombstones, and derivation-path index repair. This does not expose keyring_createAccounts yet.

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Note

Medium Risk
Touches snap persistent state and account repository behavior (batch writes and derivation-path index repair), which could affect account discovery or persistence if edge cases are missed. Changes are localized but involve data-shape tweaks (accounts tombstones) and new staged-changeset semantics.

Overview
Adds repository-level batching: BitcoinAccountRepository now supports getByDerivationPaths (single read per namespace, returns results in input order) and insertMany (writes accounts and derivationPaths in two bulk updates).

Introduces tombstone-aware state by allowing SnapState.accounts values to be null, and adds derivation-path index repair when lookups fall back to scanning accounts.

Extends BitcoinAccount with hasStaged() and updates BdkAccountAdapter to preserve/merge staged ChangeSets so batch insertion can validate staged data without consuming it; adds corresponding unit tests and updates the changelog.

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