FE-629: Fix HIR kernel compilation for conditional distributions

[ShiroKSH]

What is the purpose of this PR?

Fix buffer-ABI compilation of real transition-kernel attributes whose conditional branches produce a number and a Distribution.

Related links

• Fixes Fix HIR kernel compilation for conditional distributions #9027

Blocked by

• None

What does this change?

• Materializes mixed scalar/distribution conditionals once.
• Writes a numeric branch directly and sends only the selected distribution to the seeded RNG sink.
• Adds regression coverage for both branches and a patch changeset for @hashintel/petrinaut-core.

Pre-Merge Checklist

Has this modified a publishable library?

This PR:

• modifies an npm-publishable library and I have added a changeset file(s)

Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

Known issues

• None

Next steps

• None

What tests cover this?

• HIR unit tests cover the selected distribution sink path and scalar write path.
• TypeScript and ESLint checks pass for @hashintel/petrinaut-core.

How to test this?

1. Run yarn workspace @hashintel/petrinaut-core test:unit --run src/hir/emit-buffer-js.test.ts.
2. Confirm the conditional distribution test passes and emits one distribution sink call for the active token while writing zero for the inactive token.

Demo

Not applicable.

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PR Summary

Medium Risk
Changes stochastic kernel emission and RNG sink ordering for conditional real outputs; scope is narrow and covered by a targeted unit test, but incorrect dispatch would affect simulation sampling correctness.

Overview
Fixes buffer-ABI compilation when a real transition-kernel attribute uses a conditional whose branches are a number and a Distribution (previously this shape bailed and failed to compile).

The HIR emitter introduces a mixed value kind: conditionals where both sides are scalar, distribution, or mixed are materialized once with an if/else, and kernel output codegen dispatches at runtime—numbers go to outF64, distribution objects go to __sink("dist", …), with a TypeError if the value is neither. That keeps the seeded RNG stream aligned with the branch actually taken (inactive arms that return 0 do not register a distribution sink).

Adds a regression test in emit-buffer-js.test.ts and a patch changeset for @hashintel/petrinaut-core.

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