fix(ios): enable SQLite performance mode

[chrispader]

Summary

• compare performance_mode using its integer type
• write performance flags through s.pod_target_xcconfig
• make the two performance modes mutually exclusive

Root cause

performance_mode is initialized as an integer but was compared with string values, so neither branch executed. This also masked references to an undefined xcconfig variable inside both branches.

Impact

The configured default performance mode now applies the intended SQLite compiler options, including SQLITE_THREADSAFE=0 for mode 1.

Validation

• ruby -c packages/react-native-nitro-sqlite/RNNitroSQLite.podspec
• git diff --check