Keep save-boundary timestamps through load and restore [minor]

[matt-edmondson]

Fixes #91

What was wrong

SaveBoundary.Timestamp ("when the save boundary was created") was replaced with the current time in two of the three places the issue lists:

1. Deserialization. Timestamp was get-only and not a constructor parameter, so System.Text.Json discarded the saved value.
2. RestoreFromState. Each boundary was re-created through CreateSaveBoundary(position, description), which stamped it again.
3. Trim. This one is already fixed on main: since UndoToSaveBoundaryAsync undoes to the wrong position when given a SaveBoundary obtained before the stack was trimmed #88, AdjustPositions uses the internal copy constructor, which carries Timestamp over. This PR adds a regression test for it.

Change

• SaveBoundary: new public constructor SaveBoundary(int position, string? description, DateTimeOffset timestamp), marked [JsonConstructor]. A default timestamp means "now". That is also what STJ passes when saved data has no timestamp, so older data still loads.
• SaveBoundaryManager.RestoreSaveBoundary (internal): adds a boundary with its original timestamp and marks the initial state as not clean, as CreateSaveBoundary does.
• UndoRedoService.RestoreFromState: uses RestoreSaveBoundary when the manager is the built-in SaveBoundaryManager. Otherwise it falls back to CreateSaveBoundary.
• Docs: api-reference.md showed SaveBoundary as a record with a DateTime Timestamp, which it isn't. serialization.md's binary serializer example called a constructor that didn't exist. Both now match the real type.

Design note: why ISaveBoundaryManager is unchanged

A clean fix for (2) would add a member to ISaveBoundaryManager. The library targets netstandard2.0, which has no default interface methods, so a new member would break every external implementer. That calls for a major version bump. This PR keeps the interface as it is and special-cases the built-in manager instead. A custom manager keeps working, but its restored boundaries still get fresh timestamps. If you'd rather add something like ISaveBoundaryManager.RestoreSaveBoundary(SaveBoundary) in a breaking release, the type check in RestoreFromState can go.

The commit is tagged [minor] because it adds a public constructor.

Testing

New tests:

• JsonSerializer_SerializeDeserialize_PreservesSaveBoundaryTimestamp: a serializer-only round trip
• UndoRedoService_SaveLoadState_PreservesSaveBoundaryTimestamp: SaveStateAsync → LoadStateAsync. The saved timestamp is rewritten to a fixed past time, so a restamp cannot match by chance
• UndoRedoService_RestoreFromState_PreservesSaveBoundaryTimestamp
• JsonSerializer_DeserializeSaveBoundaryWithoutTimestamp_UsesLoadTime: older data without a timestamp still loads
• SaveBoundary_StackTrimmed_KeepsTimestamp: guards the existing trim behavior

To check that the tests catch the bug, I temporarily reinstated the old behavior (the constructor ignored the timestamp, and restore went back through CreateSaveBoundary). The three load/restore tests failed. With the fix, dotnet test UndoRedo.Test passes 105/105, and dotnet build UndoRedo builds all 7 target frameworks.

🤖 Generated with Claude Code

https://claude.ai/code/session_01TXZMdofSRgS8oXS9RgTRhM

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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