feat: add global protocol pause

[TUPM96]

/claim #42

Summary:

• Added protocol-wide pause state in instance storage with pause_protocol, unpause_protocol, and is_protocol_paused entrypoints.
• Restricted pause/unpause to the protocol admin.
• Added protocol pause checks to all external state-changing entrypoints except pause/unpause themselves.
• Emits protocol pause/unpause events and returns ProtocolPaused for blocked mutations.
• Added regression tests for admin-only controls, lifecycle mutations blocked while paused, active group mutations blocked while paused, and resumed behavior after unpause.

Validation:

• cargo fmt --check -> passed
• cargo test --package sorosave -> 12 passed
• cargo clippy --package sorosave --all-targets -- -D warnings -> passed
• git diff --check -> passed

No secrets, private runtime data, or public payment details are included. Payout details can be provided privately on acceptance.

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