feat(events): add notification priority levels (#131)

[zeroknowledge0x]

Closes #131

Summary

Adds a NotificationPriority enum (Low / Medium / High / Critical) as a trailing topic on every emitted event alongside the existing NotificationCategory. Off-chain consumers (alerting, dashboards, paging) can now route notifications by priority — or page on Critical (admin transfer, authorization failure) without decoding the payload.

Why a trailing topic

Soroban events are positional — the topic at the last index is what consumers read as the most recent routing signal. By appending priority after category, existing listeners keyed off the event name or category keep working unchanged (the extra trailing topic is simply ignored). New consumers can opt in to priority-based routing.

Semantic priority mapping

Event	Priority	Rationale
AutoshareCreated, AutoshareUpdated	Medium	Operator should track
GroupActivated, GroupDeactivated	Low	Routine lifecycle
ContractPaused, ContractUnpaused	High	Operator review required
AdminTransferred, AuthorizationFailure	Critical	Funds/security — immediate attention

Backward compatibility

• All 105 existing tests pass without modification of their category assertions.
• Helper category_of/latest_category migrated from topics.last() to topics.get(n - 2) so they keep reading the category from its (now second-to-last) position.
• New priority_of helper added for priority assertions.
• Backward-compat shape test verifies the category_topic remains stable.

Test evidence

test result: ok. 105 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 3.77s

Run: cargo test --workspace from contract/.

Files changed

• contract/contracts/hello-world/src/base/events.rs — add NotificationPriority enum, append trailing #[topic] priority to every event struct (+46)
• contract/contracts/hello-world/src/autoshare_logic.rs — set semantic priority at every publish site (+12)
• contract/contracts/hello-world/src/tests/notification_test.rs — migrate topic helpers from topics.last() → topics.get(n-2), add priority_of helper, add priority assertions (+77)

3 files changed, 126 insertions(+), 9 deletions(-)

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Signed-off-by: zeroknowledge0x zeroknowledge0x@users.noreply.github.com