ACT-Backend v1.6

Changes since v1.5, clean failures at generation:

• Removed what an earlier generation or build left behind (#60)
• Raised on a template fragment's key that is missing or unknown (#62)
• Left an instruction with no semantics out of the backend (#64)
• Refused a spec name the generated backend cannot carry (#66)
• Refused an operation the selector lacks and mapped concatenate (#69)
• Refused the semantics values a rule cannot be written from (#74)
• Refused the inputs and attributes a rule cannot be written from (#79)
• Wrote an empty access list and refused unaddressable operands (#83)
• Let the search run ten times as long as its best candidate took (#85)
• Described a cost model as evaluating a candidate (#86)
• Cited the OOPSLA 2026 paper in place of the arXiv preprint (#87)

This release:

• The version is 1.6 in pyproject.toml, in act_backend/__init__.py and in the README's install line, and 1.6.0 in the generated crate's Cargo.toml and Cargo.lock.
• Generation now raises ValueError, naming what is refused, where it built a backend that panicked, misaddressed an operand or failed in the build: for a name the generated code cannot carry, an operation or a value count the selector lacks, a value or an input a rule cannot be written from, and an operand the allocator cannot address.
• A size written with // is refused by name where its build failed: the division a size is written with is /.
• A semantics may use concatenate, an instruction with no semantics is left out of the backend, a generation that fails leaves no earlier binary, and a build by hand makes the allocator's build tree anew.
• A compile runs ten times as long as it took to find its best candidate, where it ran twice as long, so it evaluates more of a kernel's candidates.
• This release still pins TAIDL v2.3, and QKV and QKV_new compile to the assembly they compiled to at v1.5, byte for byte.
• Its 170 tests run in devanshdvj/act-env:py3.14-jax0.10.2 on TAIDL-TO v2.5, on amd64 and arm64, on the installed wheel.