Fix automatic lifting through extension chains

[jasonhemann]

Hi. My friend Claude and I fixed this using the conservative per-language extension counter approach we discussed in #2.

This commit makes 3 fixes:

• Explicit extensions remain in exts, while automatically generated lifts are cached separately with the current value of the target language's extension counter. Parameter resolution still prefers an exact explicit extension. Otherwise, parameter resolution reuses a cached automatic lift only when the counter value recorded with the lift matches the target language's current extension counter; registering an explicit extension advances the corresponding language's extension counter.
• Extended objects no longer capture their already-lifted base when first defined. Whenever an extended object is reconstructed for a target language, the object's originally declared immediate base is automatically lifted to that target language. Base reconstruction uses automatic lifting directly, since consulting explicit extensions while rebuilding the base could select the descendant currently being reconstructed.
• Explicit extensions are now registered through the stored base chain, so lookup through any ancestor sees the new descendant. The language generation still advances only once per explicit declaration.

[camoy]

Thanks!