Add root mutation detection and shared test helpers

[jmalloc]

Add mutation detection for aggregate and process root state within handler scopes. The scope now panics with a descriptive UnexpectedBehavior error when mutations are detected outside of sanctioned operations (e.g., mutating root state between handler method calls).

Changes

• Mutation detection: Aggregate and process scopes now snapshot root state and detect unsanctioned mutations, reporting the handler identity and source location
• internal/compare package: Deep equality checks using unsafe reflection, extracted for reuse across scope implementations
• Shared test helpers (xtesting): ExpectPanicMatching (generic panic recovery with type assertion and match callback) and ExpectLocation (source location assertions)
• Test cleanup: Eliminated package-local test helpers in aggregate and process packages, replacing them with shared xtesting functions
• Missing tests: Added mutation detection tests for process scope (before ScheduleDeadline, between two scope calls, at end of handler without scope call)
• Naming: Renamed test functions and fixtures for clarity (dropped redundant "Scope" prefixes, standardized fixture variable names)

[Copilot]

Pull request overview

Adds root mutation detection for aggregate and process handler scopes, backed by a shared comparison helper and reusable testing assertions.

Changes:

• Introduces internal/compare with unsafe reflection support for comparing root/message state, including unexported fields and function values.
• Adds aggregate/process shadow-root checks to detect direct mutation and non-deterministic sanctioned mutations.
• Refactors tests to use shared xtesting panic/location helpers and adds mutation-detection coverage.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test.go	Uses shared comparison helper for fact value matching.
compare.go	Delegates default message comparison to shared comparison helper.
fact/aggregate.go	Adds snapshot offset metadata to aggregate loaded facts.
internal/compare/doc.go	Documents the new comparison package.
internal/compare/compare.go	Implements shared deep equality logic.
internal/compare/compare_test.go	Adds comparison behavior tests.
internal/compare/internal/unsafereflect/value.go	Adds unsafe reflection utilities for unexported fields.
internal/compare/internal/unsafereflect/value_test.go	Tests unsafe reflection helper behavior.
internal/compare/internal/unsafereflect/LICENSE.credits	Adds attribution for unsafe reflection approach.
internal/x/xtesting/expect.go	Adds shared panic and location assertion helpers.
engine/internal/aggregate/controller.go	Builds shadow aggregate roots and validates snapshot/event divergence.
engine/internal/aggregate/controller_test.go	Updates aggregate controller tests for shared helpers and snapshot behavior.
engine/internal/aggregate/scope.go	Adds aggregate mutation detection and non-deterministic ApplyEvent detection.
engine/internal/aggregate/scope_test.go	Adds aggregate mutation detection tests.
engine/internal/process/controller.go	Builds process shadow roots and checks for end-of-handler direct mutation.
engine/internal/process/controller_test.go	Updates process controller tests to shared helpers.
engine/internal/process/scope.go	Adds process mutation detection and non-deterministic Mutate detection.
engine/internal/process/scope_test.go	Adds process mutation detection tests.

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