test: lock v1 wire shapes and public API; document deferred surface

[joshua-temple]

What this change does

Prepares the state 1.0 stability contract:

• Wire + API freeze test. Extends the freeze tests beyond the five host interfaces they pinned, locking the serialized shapes and the core public signatures the 1.0 promise rests on. TestWireShape_Frozen reflects over the 10 serialized types (IR, State, Transition, Region, Invocation, Ref, ContextSchema, Descriptor, ParamSpec, BindingSpec) and pins their exported field names and json: tags (81 fields); compile-time assertions pin 10 load-bearing signatures (Forge, ForgeFor, Quench, Cast, Fire, LoadFromJSON, ToJSON, Registry.Palette, ToMermaid, ToDOT), each ending in its option tail so additive options need no edit. A rename or retag breaks the test; an additive field is a deliberate, documented test update.
• Deferred-surface docs. Records, in the changelog and godoc, the features deliberately left for post-1.0 as additive (non-breaking) work — payload-carrying Raise, multi-target transitions, context.Context on the non-Fire entry points, relaxing the non-quiesced snapshot refusal, finer Inspector cadence, and a metrics Meter seam — each with the additive mechanism it will arrive through. This marks them as intentional boundaries, not oversights.

Why

Freezing a contract is only safe if a test catches accidental breaks and the deliberate boundaries are written down. This makes the 1.0 wire/API shape enforceable in CI and the deferred surface honest.

Relates to: 1.0 stability hardening

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• mage check passes locally — state passes lint, race tests, and govulncheck; the only mage check failure is the pre-existing telemetry/datadog stdlib vulnerability, unrelated to this change.
• Tests added/updated for the change
• Public API changes documented (godoc) and noted for the changelog