feat(core): 领域模型、能力接口、SPI 与同步引擎骨架
- 标识体系：AccountKey(provider+platform+omsStoreId)、CanonicalKeys
三级标识（canonicalKey 不含 provider）与四类指纹
- 模型：Platform/ChannelProvider/Capability/FulfillmentMode、
Money(BigDecimal+Currency)、PlatformTime 三轴、SalesOrder/OrderLine/
ProductListing/Page/Query
- 能力接口：PlatformAdapter、AggregatorCapable、SalesOrderCapable、
ProductCapable
- 异常：ChannelException 基类 + Auth/RateLimit/Transient/Validation/
PlatformBusiness/IdempotencyConflict/CrossProviderConflict/Unsupported
- SPI：ChannelAccountRepository、SecretProvider、TokenStore、
SyncCursorStore、Sink、LockProvider（均只定义，宿主实现）
- SyncEngine：from = watermark - overlap，整批成功才推进水位线
Co-Authored-By: Claude <noreply@anthropic.com>
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chenchuchuan