Fix failing importData tests from PR #678 (IndexedDB support)

[Copilot]

PR #678 refactored importData to require a versioned export format (__meta__ + typed namespaces), but the tests still passed raw chrome.storage data directly, causing two failures: merge rules returning [] and allowDelay being undefined instead of false.

Infrastructure added

• src/util/guard.ts — isRecord, isOptionalInt, isVector2 type guards via typescript-guard
• src/database/common/migratable.ts — isExportData, isLegacyVersion, extractNamespace for versioned import/export logic
• src/database/types.d.ts — BrowserMigratable<N> interface for databases that support import/export
• types/timer/backup.d.ts — ExportMeta and ExportData types for the versioned format

Database updates

merge-rule-database and limit-database now route importData based on version:

async importData(data: unknown): Promise<void> {
    if (isLegacyVersion(data)) {       // version.__meta__.major < 4
        return this.importLegacyData(data)
    }
    const rules = extractNamespace(data, this.namespace, createArrayGuard(isMergeRule)) ?? []
    await this.add(...rules)
}

Legacy path preserves the original key-based chrome storage import. Both databases also expose exportData() implementing BrowserMigratable.

Test fix

Added test/database/migratable.ts with mockLegacyData to wrap raw storage snapshots with __meta__: { version: "3.8.15" }, routing them through the legacy import path:

await db.importData(mockLegacyData(data2Import))  // was: db.importData(data2Import)

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