docs(registry): document the keys_xdr encoding a manifest must use

[0dillon]

Summary

RegistryEntry.keys_xdr holds XDR-encoded ScVal ledger keys and is never decoded on-chain. The
repository documents that rule but never shows an adopter how to produce the values. Working them
out currently means writing a throwaway test that runs ScVal::try_from_val and to_xdr and
prints the hex — which is what produced the three constants in scripts/init_testnet.sh.

That is a real barrier to adoption: the first thing a new contract has to do is the one thing
nothing explains.

Acceptance Criteria

• Documentation covers what each entry in keys_xdr is: the XDR of the ScVal form of a
  storage key, hex-encoded.
• It shows the instance entry case (ScVal::LedgerKeyContractInstance, 00000014).
• It shows a #[contracttype] unit enum variant case and explains why it encodes as
  ScVal::Vec([Symbol(name)]).
• It shows a reproducible way to generate the hex for an arbitrary key.
• The three constants in scripts/init_testnet.sh are cross-referenced as a worked example.
• It restates that the manifest is advisory and can drift from the compiled macro keys.

Tech Stack

Rust (edition 2021, toolchain 1.93.0 pinned in rust-toolchain.toml), soroban-sdk 27.0.4,
target wasm32v1-none. Build with stellar contract build, never cargo build. Tests are
#[cfg(test)] modules on Env::default(); run them with cargo test --all.