feat: define the 0.4.0 engine job kernels

[0xgleb]

Introduce the pure engine crate and its typed job record model. Implement the normative K1 and K3-K9 decisions, including check order, token fencing, checked counters, claim-budget abandonment, defer checkpoints, and explicit changed-versus-unchanged outcomes. Cover the contract with focused kernel tests.

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This is part 4 of 6 in a stack made with GitButler:

•  6  feat: define the 0.4.0 storage schema #21
•  5  feat: encode engine job records as deterministic CBOR #20
•  4  feat: define the 0.4.0 engine job kernels #19 👈
•  3  chore: update 0.4.0 ownership metadata #18
•  2  build: unify the Rust and Haskell toolchain #17
•  1  docs: specify the 0.4.0 monorepo engine #14

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