Add occurrence param to Spec.findEvent

[Ryang-21]

What

Spec.findEvent now takes an optional occurrence — a 0-based index among
same-named events, in declaration order — and passes it through to the
underlying implementation, which already accepted one. Defaults to the first
declaration, so existing calls are unchanged. A non-integer or negative
occurrence throws; an index past the last declaration returns undefined,
the same as an undeclared name.

Also updates the v16.2.0 changelog entry for events to cover #1572, which
landed after the release commit: duplicate event declarations, the
occurrence param on eventTopicFilter and findEvent, and the new
stellar-sdk bindings warnings for duplicate and renamed generated names.

Why

#1572 added support for contracts that declare the same event name more than
once — composed modules each emitting their own transfer. It threaded
occurrence through the internal findEvent and through
Spec.eventTopicFilter, but not through the public Spec.findEvent. So a
caller could build a filter row for the second transfer declaration but
could not read its spec: findEvent always returned the first.

v16.2.0 is cut but not tagged, so shipping this now keeps the whole
duplicate-event feature in one version instead of leaving a released API
half-finished.

[Copilot]

Pull request overview

Exposes duplicate-event selection through the public Spec.findEvent API.

Changes:

• Adds the optional, validated occurrence parameter.
• Adds duplicate-event and validation tests.
• Updates generated reference documentation and the v16.2.0 changelog.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/contract/spec.ts	Forwards occurrence to the existing implementation.
test/unit/spec/event_spec.test.ts	Tests selection, defaults, misses, and invalid indexes.
docs/reference/contracts-client.md	Updates generated API reference.
CHANGELOG.md	Documents duplicate-event support.

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