Add emergency pause controls

[Sikkra]

Summary

• Store an emergency pause admin and paused flag in persistent strategy storage.
• Add admin-gated pause/unpause entrypoints, get_admin, is_paused, and a typed pause_state contract event.
• Block new deposits and harvest re-looping while paused, while leaving withdrawals available.
• Cover admin pause/unpause, non-admin rejection, and paused deposit/harvest guards in unit tests.

Closes #45

Verification

• PASS: CARGO_TARGET_DIR=%TEMP%\codex-cargo-target-blend-pause cargo test pause -- --nocapture --test-threads=1
• PASS: git diff --check
• BLOCKED locally: cargo test --lib -- --test-threads=1 reaches 30 passed / 16 failed, but the failures are Soroban snapshot writes to ignored test_snapshots\*.json paths returning Access is denied in this Windows workspace.