Guard BtShared.nRef reads with SQLITE_OMIT_SHARED_CACHE - #628

SQLCipher 5.0.0-beta.1 no longer compiles with SQLITE_OMIT_SHARED_CACHE. The shared-cache fix in 1825a0a reads BtShared.nRef in the cipher_migrate refusal and in an assert in sqlcipher_ctx_set_error, and SQLite declares nRef only when shared cache is compiled in. Builds that omit shared cache, such as the sqlite-wasm-rs Wasm build, stop with 'BtShared' has no member named 'nRef'.

Both reads now sit under #ifndef SQLITE_OMIT_SHARED_CACHE, the same guard SQLite puts around the field. Without shared cache Btree.sharable is never set, so ctx->shared stays 0 and neither check could fire. Builds with shared cache compile exactly the code they did before.

The shared-cache tests in the core, compatibility and rekey suites are skipped with ifcapable shared_cache when shared cache is omitted, since a cache=shared URI then opens a private cache and those tests cannot hold. That lets the SQLCipher suite run on a build without shared cache.