Read FROST shares stored before group subkeys

[kwsantiago]

FROST shares stored by any earlier release could not be read: keep frost list and every command that loads a share failed with Serialization error: tag for enum is not valid. #963 added group_subkey_secret to ShareMetadata, which is stored with bincode inside the KSH1 v1 blob. bincode is positional, so #[serde(default)] cannot supply the missing field, and older blobs were read with the bytes after did_backup shifted.

• New writes use format version 2, the current layout.
• v1 and unprefixed blobs are read through a frozen copy of the pre-change ShareMetadata, with no subkey.
• The legacy-blob tests built their "old" blobs from the current struct, which is why they passed; they now use the frozen layout. New tests cover a v1 blob (every metadata field and the ciphersuite carried over) and a subkey round trip in the current layout. The v1 test fails with the old decoding.

Checked with the released binaries: vaults written by v0.7.5 and v0.9.2 fail to list their shares on main, and with this change list them, sign with them, and the signatures verify. KeyRecord, SecretRecord, ThresholdSeal and StoredShare itself are unchanged since v0.9.2, and the other vault records are JSON.

Shares written by this version are v2, which earlier releases cannot read.

[coderabbitai]

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