Regenerate with ids on contract spec entries

[leighmcculloch]

Note

Part of a stack of PRs that must merge in this order.

A first group of PRs deliver const-encoded contract specs, so that contract specs are produced at compile time instead of at proc-macro execution time. This provides the foundation for the capability to construct the specs from information that is not known at proc-macro execution and only known at compile time, like the fully qualified name of a type:

1. Add borrowed Ref variants of generated types #560
2. Add const XDR serialization on View types #562
3. Encode contract spec XDR at const evaluation time rs-soroban-sdk#1965

A second group of PRs deliver ids on contract spec entries and in spec references. Every entry carries an 8-byte id hashed from the fully qualified name of the item it describes, and a reference to a user-defined type carries the id of the entry it refers to, so references stay unambiguous even when items share a name. This resolves the type identity problem (stellar/rs-soroban-sdk#1570) and type alias limitations (stellar/rs-soroban-sdk#1857):

1. Add ids to contract spec entries stellar-xdr#313
2. Regenerate with ids on contract spec entries #568 ← this PR
3. Add ids to contract spec entries rs-soroban-sdk#1998
4. Adopt spec entry ids and id-based reference resolution stellar-cli#2677

What

Regenerate the Rust types from stellar/stellar-xdr#313: a new ScSpecEntryV2 struct pairing an 8-byte id with a ScSpecEntryV2Body union that reuses the existing V0 entry bodies, exposed as a new ScSpecEntry::V2 arm; a ScSpecTypeUdtv2 reference carrying the referenced type's name and the id of the entry it refers to; and the SC_SPEC_ID_LEN const. All new types come with the const-writable View counterparts, so the SDK's proc macros can encode them at compile time.

Why

Ids on spec entries identify each entry exactly — the id is the truncated SHA-256 of the fully qualified name of the item the entry describes — so every entry has a unique identifier, and a reference carrying an id refers to one entry precisely, even when entries share a name.