State the real text-form contract for wrapper sets, and what isn't checked yet #59

The docs promise a check that the code doesn't make. docs/value-sets.md#the-text-form-contract says:

A decorative format ("CUST-{value}") fails loudly at the persistence boundary with an error naming the contract.

For the core wrapper arities that's only true when the decoration is something the primitive's grammar rejects. A decorated string, or a number that's simply wrong, is written without complaint, and can come back as a different element (#53).

For 9.1.0, state the contract as it really is and say what isn't checked yet:

• The contract: an element's ToString(format, invariant) and Parse(text, invariant) must be exact inverses. Parse(element.ToString()) returns an element equal to the original, and the text is the backing primitive's own form.
• What's checked today: the five NodaTime arities check the round trip. The integer, decimal, Guid and temporal arities reject only text their primitive can't parse. StringSet<T> checks nothing, because any text is a valid string.
• What breaks if it's violated: data that can't be read back, or a value that changes on every round trip, with no error.
• The fix: 10.0.0 checks the round trip for every arity (Wrapper set arities don't check that an element's text form round-trips #53).

Pages: docs/value-sets.md (the contract section and its table), docs/architecture.md (the text-form contract paragraph and the BridgedElementTypeMapping bullet, which says the contract "fails loudly here"), and the value set XML docs if they repeat the promise.

Coordination:

• Adopt the plain-docs voice and rewrite the consumer docs in it #38 rewrites docs/value-sets.md in the plain-docs voice, so write this on top of it.
• Claim S07-decorative-format-fails-loudly in test/oracles/claims.json quotes the sentence above. Re-anchor it to the corrected text, and keep the Wrapper set arities don't check that an element's text form round-trips #53 candidate rows registered: the behaviour stays wrong until 10.0.0.