Scope SQLite control argument lifetime

[birdmanmandbir]

What changed

• Keep borrowed control arguments bound with STATIC; SQLite does not need to copy them.
• Mark the binding helper unsafe and document its lifetime requirement.
• Explicitly finalize the statement before its owned control argument leaves scope.

Root cause

The owned argument was declared after the statement, so normal Rust drop order could destroy it first. STATIC bindings require the backing value to outlive the statement.

Validation

• cargo fmt --all -- --check
• cargo test -p powersync --all-features
• cargo clippy -p powersync --all-targets -- -D warnings

Fixes #35

[simolus3]

I don't agree with this, and the test doesn't reproduce how PowerSyncControlArgument is actually used in invoke_control.

Semantically, we want to borrow the string to the SQLite statement, and the static destructor is acceptable for this. To make this cleaner rust, we could

1. Mark bind_to as unsafe, and add a safety comment stating that the reference must outlive the statement.
2. Explain that in invoke_control, where we'd explicitly drop the statement before arguments.

[birdmanmandbir]

Addressed in b0c1606: this keeps borrowed STATIC bindings, makes bind_to unsafe with its lifetime contract, and explicitly drops the statement before the argument can leave scope. I also removed the regression test because it bound a temporary rather than exercising invoke_control's real lifetime.

cargo fmt --all -- --check, cargo test -p powersync --all-features, and cargo clippy -p powersync --all-targets -- -D warnings pass.