toon: decode raises ValueError where the contract is ToonDecodeError #198

What

Nine decode conformance cases raise ValueError where the module's contract is ToonDecodeError:

decode/validation-errors[4]   raised ValueError (want ToonDecodeError)
decode/validation-errors[5]   raised ValueError
decode/validation-errors[6]   raised ValueError
decode/validation-errors[80]  raised ValueError
decode/validation-errors[87]  raised ValueError
decode/validation-errors[94]  raised ValueError
decode/validation-errors[95]  raised ValueError
decode/validation-errors[96]  raised ValueError
decode/validation-errors[101] raised ValueError

The cases are malformed input that decode() should reject, so raising is right — but with the wrong type. A caller writing except ToonDecodeError (which is what the module's own docstring and Raises: sections promise) does not catch these; they escape as a bare ValueError.

Found while reconciling numbers for #195: the conformance gate requires ToonDecodeError specifically on the 158 shouldError decode cases, which is the stricter and correct reading. Under that rule the module is at 225/404 on decode, not 234/404 — my earlier figure on #184 accepted any exception. All nine are currently inside toon/conformance_baseline.json, so the gate tolerates them; that is the ratchet working as designed, not an endorsement.

Fix

Make the decode paths that raise bare ValueError raise ToonDecodeError instead — likely the same places that treat validation failures as generic value errors. Then shrink the baseline by those nine IDs, which is the mechanism #195 provides.

Worth doing while the module's version is already moving for the spec work, since it is an API-visible contract rather than a spec gap.