[RaymondAbiola]

Summary

Closes #1, #2, #3, #4

Issue #1 — Doc comments on all public functions

• Added /// doc comments to every pub fn in the #[contractimpl] block
• Each comment describes what the function does, who may call it, and which Error values it can return

Issue #2 — AdminChanged event on set_admin

• Added AdminChanged { old, new } struct to events.rs
• set_admin now emits the event so indexers can track admin transfers
• Tests added: set_admin_emits_admin_changed_event, set_admin_updates_stored_admin

Issue #3 — Exact-goal boundary donation tests

• Added three_donors_reach_goal_exactly_fourth_fails: three donors hit goal exactly, fourth attempt returns GoalExceeded
• Added two_donors_reach_goal_exactly_campaign_fully_funded: verifies milestone can release when goal is exactly met

Issue #4 — Verifier reassignment

• Added reassign_verifier(campaign_id, new_verifier) callable by admin only
• New verifier must be registered; campaign must be active
• Emits VerifierReassigned { campaign_id, old_verifier, new_verifier }
• Tests: happy path, unregistered verifier, cancelled campaign, expired campaign, new verifier can approve after reassignment
• ARCHITECTURE.md events table and design notes updated

Also fixed (pre-existing breakage)

• Duplicate error discriminants in errors.rs (prevented compilation)
• Unclosed delimiter in pull_pledges, duplicate imports and double transfer in approve_milestone
• Broken test_measure_resource_costs (deprecated env.budget() → env.cost_estimate().budget())
• Broken test_exact_events_emitted (invalid SDK APIs) → replaced with test_events_emitted

Tested

make test — 31 tests pass
make lint — no warnings or errors

[sweetsoul21]

Goood job @RaymondAbiola