refactor: rename state Assay check to Verify

[joshua-temple]

What this change does

Renames the state engine's trust-boundary check from Assay to Verify (pre-v1; latest released tag is state/v0.2.0):

• Machine.Assay(state, entity) → Machine.Verify(state, entity), with AssayError → VerifyError and AssayOption → VerifyOption.
• Foundry vocabulary doc moves Verify from the metaphor verbs to the plain, discoverability-first verbs (alongside PlanPath, Requirements, Trace). The other foundry verbs (Forge, Temper, Quench, Cast, Fire) are unchanged.
• Docs page authoring/assay.md → authoring/verify.md (+ slug/cross-link + the verify-gate brand image), CHANGELOG bullet under the staged [1.0.0] section.

Why

Assay is an uncommon word, and even knowing it ("test a metal sample") it doesn't signal what the call does: confirm an externally-hydrated entity legally satisfies a state's requirements. Verify says exactly that and pairs with the already-plain Requires/Requirements. This follows the package's own stated rule: operations that favor discoverability over metaphor stay plain. Doing it now, before the state 1.0 tag, keeps it out of the frozen stable surface.

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• state, source, and source/statemachine build/test/vet green; docsgen clean
• Public API change documented (godoc, docs site, CHANGELOG)