[TS] Add private assume for source semantic models

[CaelmBleidd]

Adds a private TsModelPrimitives.assume(condition: boolean): void intrinsic for source-model artifacts. It accepts only a same-artifact, unnamespaced helper call from a private source-model caller while that artifact has an active dispatcher-owned frame. Ordinary application declarations remain unknown calls. Model authors select the existing catalog dependency ts.model.assume; there is no public application API or new model framework.

The positive Boolean guard is the only continuation. False/inconsistent assumptions prune the state without FRESH completion, unsupported-prefix reporting or engine failure. The dispatcher now treats a model relation with no feasible successor as pruning instead of throwing. Existing fork machinery refreshes cached solver models. Evaluating a TypeScript short-circuit argument can still branch before the helper call; this bridge does not rewrite arbitrary source expressions.

Validation: six focused assumption tests plus five existing stopped-prefix tests pass, including constant true/false, symbolic guard, deliberately incompatible cached models, application spoofing, namespace ownership and absent active frame. Positive numeric witnesses replay against the original identity-call contract in Node. Strict usvm-ts detektMain/detektTest passed. Namespace and inactive-frame ownership checks deliberately exercise the low-level dispatcher contract; the assumption execution tests use ordinary TypeScript source calls. No new production model is made approximate or domain-restricted by this PR.

Depends on #528 for the source artifact ownership boundary; its review diff is based on that branch. The no-survivor dispatcher hunk is separate from the Date residual-receiver preservation work and from #530's per-helper fallback policy.