lakefile: drop Blake3 from the native-decide dynlib

[samuelburnham]

Blake3 now precompiles its libraries, so Lake loads their shared objects -- which bundle the C and Rust FFI objects -- into any process elaborating a module that imports them. The Blake3 half of ix_native_decide_dynlib was assembling that by hand from a blake3_rs_shared cdylib, and that target no longer exists upstream.

The target keeps Ix's own externs, which nothing else supplies. Precompiling Ix.Unsigned instead would work, but only as its own library declared after Ix: both would claim the module, Package.findModule? resolves with findSomeRev?, and losing that race silently stops precompiling it -- with the symptom appearing as a missing native implementation inside a proof file rather than as a configuration error. A local dynlib naming its modules outright is worth more than the lines it costs.

The Blake3 pin moves to the revision that turned precompilation on, and moves in lakefile.lean, lake-manifest.json, flake.nix and flake.lock together. The lakefile drops the cdylib that the older revision still provides, so a pin left behind in any one of them pairs the new lakefile with a Blake3 that does not precompile -- and that mismatch surfaces as a missing native implementation inside a proof file rather than as a build error.