feat!: separate swap-guard restriction from the generator verdict

[mfw78]

• PollResult gains a registry-level Restriction overlay (NONE | SWAP_GUARD), populated in the shared _poll so checkOrder reports it too
• the generator verdict is never overwritten: a guarded POST stays visible, the signature is withheld
• INVALID becomes uniformly terminal as an invariant — no reason-selector branching for control flow; unpark is event-driven via the existing SwapGuardSet (clear = address(0))
• SwapGuardRestricted remains the settlement-path revert

BREAKING: PollResult shape changes.

Issues

Closes #32.

Part of #1.

[github-actions]

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