feat(map): surface AppMap.Monument.is_custom_name

[HandyS11]

Summary

Applies the one wire change reported in #138 for server build 25653776: Facepunch added optional bool is_custom_name = 4 to AppMap.Monument.

• Proto: AppMap.Monument.is_custom_name = 4, copied exactly from the drift report.
• ServerMapMonument.IsCustomName (bool?): mapped through ShouldSerializeIsCustomName() rather than read directly. A server older than this build leaves the field out, and that maps to null instead of false. Same shape as ServerInfo.CamerasEnabled (feat(team): add KickFromTeamAsync + surface AppInfo.cameras_enabled #124).
• Mapper comment: no longer claims token is always a localization key.

The change only adds a field, so current library builds still parse AppMap from new servers; they just can't see the flag.

Unverified assumptions

These weren't checked against the decompiled server; only the drift diff was available:

• What the field means: read from the field name as "token is a literal custom name rather than a monument translation key". The XML doc for IsCustomName says this.
• When null appears: assumes current servers always send the field, so null means "server too old". If the server only sends it when true, null would also cover "not custom".

Test plan

• New RemainingMapperTests: true / explicit false pass through; field missing gives null
• Missing-field test confirmed to fail against the naive IsCustomName = appMapMonument.IsCustomName mapping
• dtk dotnet build RustPlusApi.sln (strict analyzers) clean
• dtk dotnet test RustPlusApi.sln: 1245 passed on both net8.0 and net10.0 hosts
• ReSharper ReformatAndReorder applied to the changed files

Closes #138

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟢 Approval recommended

The one-field addition faithfully replicates the reviewed CamerasEnabled pattern, matches the drift report exactly, and has full branch coverage with no objective defects.

Review effort: Balanced
Findings: None

What changed in this PR

This PR surfaces a single new wire field that Facepunch added to the Rust+ companion protocol in server build 25653776 (reported in #138): optional bool is_custom_name = 4 on AppMap.Monument. The flag tells consumers whether a monument's token/Name is a server-defined custom name rather than a built-in monument localization key. The implementation mirrors the previously reviewed ServerInfo.CamerasEnabled pattern (#124): the field is modeled as a nullable bool? and mapped through protobuf-net's ShouldSerialize* presence check so that an older server omitting the field maps to null rather than a fabricated false.

Changes:

• Added is_custom_name = 4 to the AppMap.Monument message in the authoritative proto.
• Added ServerMapMonument.IsCustomName (bool?) and mapped it via ShouldSerializeIsCustomName(); refreshed the mapper comment about token.
• Added unit tests covering the present (true/false) and absent (null) cases.

File	Description
src/​RustPlusApi/​Protobuf/​RustPlusContracts.proto	Adds the new optional bool is_custom_name = 4 field, copied verbatim from the drift report.
src/​RustPlusApi/​Data/​ServerMapMonument.cs	Adds the nullable IsCustomName property with XML docs matching the CamerasEnabled style.
src/​RustPlusApi/​Extensions/​AppMapToModel.cs	Maps the field via ShouldSerializeIsCustomName() presence check; updates the token comment.
tests/​RustPlusApi.UnitTests/​RemainingMapperTests.cs	Adds tests pinning true/false pass-through and null-on-absent, covering both mapper branches.

The change is small, correctly scoped, and faithfully mirrors the established CamerasEnabled precedent. The proto edit matches the drift diff exactly, the mapper uses the same ShouldSerialize* presence pattern as AppInfoToModel.cs:28-30, the model docs match ServerInfo.cs:51-55, and both mapper branches are exercised by the new tests. The semantic ambiguity around what null means is the same accepted tradeoff as CamerasEnabled and is explicitly documented by the author. I found no objective defects.

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