test(registry): page at the limit with maximum-size manifests

[0dillon]

Summary

page returns whole RegistryEntry values, each carrying its full keys_xdr vector. The guard on
limit bounds the number of entries but not their size, so a page of entries with large manifests
can exceed a transaction's read limits. A keeper hitting that gets a failure the registry never
warned about.

The point of this issue is to find where the ceiling actually is, not to guess at it.

Acceptance Criteria

• A test registers entries with realistically large keys_xdr vectors and pages through them at
  the maximum permitted limit.
• The ledger-read budget the call consumes is measured, not assumed.
• The result is written into page's rustdoc as concrete guidance on choosing limit.
• If the maximum limit can exceed the budget, a follow-up issue is opened for the fix — this
  issue only establishes the number.

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.