refactor: move event handling interface to core

[adr1anh]

Describe your changes

Moves the event-handler interface and declarative advice mutations into miden-core so libraries can define handlers without depending on miden-processor.

• Adds the handler-facing EventContext with explicit event identity, active-context memory access, idiomatic accessor names, and documented snapshot allocation.
• Moves ExecutionOptions into miden-core while preserving the miden-processor re-export.
• Moves HostLibrary into miden-mast-package.
• Keeps common processor handler implementations working through deprecated compatibility accessors isolated in a documented compatibility module.
• Updates built-in core-library handlers and processor hosts to the new interface.
• Makes EventContext usable across await points by requiring Sync providers.

Migration

The generated documentation for miden_core::events::compatibility contains the migration guide, old-to-new accessor mapping, intentional differences, and before/after examples.

Validation

• Workspace clippy passed with warnings denied.
• Workspace formatting check passed.
• Affected crates passed cargo check.
• miden-core: 224 tests passed.
• Processor event-handler compatibility and async-host tests: 4 tests passed.
• Full heavy local integration testing was intentionally not rerun; this is a draft PR and CI will provide the broader sweep.

Checklist before requesting a review

• Branch created from next according to naming convention.
• Commits are signed and follow project conventions.
• Tests added or updated for the changed interface.
• Documentation and migration guidance updated.
• No changelog required for this internal refactor.