refactor: replace merge.ts tauriConf: any with PakeTauriConfig

Introduce a shared PakeTauriConfig that composes the existing PakeConfig
(now including the optional inject field) with narrow typings for the
bundle and app subtrees we actually touch. Replace all six tauriConf
parameter signatures in bin/helpers/merge.ts, narrow the platform-map
key cast where PlatformSpecific is indexed, and hoist the Linux bundle
into a local so the non-null assertion stays at the entry of the
platform-guarded helper instead of being repeated at every access.