FlatPPL alpha 7

This alpha includes upstream compiler updates through 5aaf264b and the fork's
scan fixes. Reshape propagation preserves shared scan expressions, and scan
fission runs after differentiation. The fixes remain separate feature commits.
Constant-index update padding stays static, so batched integration gradients
remain compatible with XLA after the upstream changes.

Wheels cover Python 3.12, 3.13 and 3.14 on macOS arm64, Linux x86_64 and Linux
aarch64. JAX and jaxlib remain pinned to 0.11.2.

Validation: all nine native ABI loads pass. The native suite passes 1,537 tests
with nine expected failures. M4 passes 284 Python tests; A100 passes 70 batching,
transform and integration tests. Eleven derivative tests pass on M4, A100, GH200
and GB10. Float32 and float64 scan values, JVPs and gradients match the oracles.

On M4, four-lane float64 ARMA value-plus-gradient falls from 81.7–82.9 us to
5.49 us per invocation in a compiled loop (native JAX: 5.35 us). GPU gains depend
on the recurrence and device. The variance recurrence still trails JAX on CPU.

provenance.json records source revisions, the identical build/release tree,
and wheel/native hashes. SHA256SUMS lists the wheel checksums.