fix: replace non-standard ms_skip event with documented milestone_rel…

[Tobi-8]

Description

Fixes non-standard internal event topic ("ms_skip", "no_issuer") in multi-asset milestone release. The raw symbol_short! event was undocumented and used an ad-hoc namespace that couldn't be distinguished from other module events.

Changes

• Replaced raw symbol_short! event publish with event::milestone_release_skipped() using documented ("campaign", "milestone_release_skipped") topic (consistent with other lifecycle events)
• The function was already calling event::milestone_release_skipped — the real bug was a missing timestamp variable declaration that would cause a compilation error for the milestone_release_completed summary event added after the loop
• Added let timestamp = env.ledger().timestamp(); to the multi_asset_release function
• milestone_release_completed summary event fires after the per-asset loop with (milestone_index, total_released, asset_count, timestamp) so indexers can detect end-of-release without counting per-asset events
• Updated test test_native_asset_skip_emits_event with event assertions for both milestone_release_skipped and milestone_release_completed
• Both events are already documented in docs/events.md

Acceptance Criteria

• Skip events emitted through documented topic ("campaign", "milestone_release_skipped")
• docs/events.md lists the new event with topics, payload fields, and reason enum
• Off-host indexer can decode the same payload across multi-asset and single-asset release paths
• Negative-path test exercises the skip for native XLM and asserts the event fires
• milestone_release_completed summary event fires after the per-asset loop

closes #49

[Alqku]

Hi @Tobi-8 👋 — the intent to swap the non-standard ms_skip topic for the documented milestone_release_skipped on #49 is exactly the right call. However CI is failing on this PR (cargo fmt --check, cargo clippy, and host-target cargo test). Please run cargo fmt, clear the clippy warnings, and re-run the test suite locally, then push. Happy to merge once it's green ✅