Adopt Auto Lambda syntax in Builder code guidance and Copilot

[nighca]

Background

XGo now supports Auto Lambda: classfile commands explicitly opted in through gox.mod can use a trailing block as the final lambda argument without the => boilerplate. For example, onStart { ... } and times 3 { ... } are shorter, equivalent spellings of the existing explicit-lambda forms.

For applicable Builder/SPX commands, this is a clearer and more approachable style for users writing game code. Builder should guide users toward it rather than continuing to foreground the more verbose spelling.

Scope

Adopt Auto Lambda as the preferred example style wherever the command and argument count have been enabled for it.

• Update Builder-maintained API-reference examples and code snippets.
• Update editor completion snippets and related code-generation templates.
• Ensure Copilot understands Auto Lambda, uses it in generated examples when applicable, and encourages the style in its guidance.
• Audit relevant documentation, tutorials, and built-in skills/prompts for examples that should be migrated.

Constraints

• Auto Lambda is classfile- and command-specific. Do not suggest it for commands that are not explicitly enabled, or in ordinary non-classfile XGo source.
• The existing explicit => syntax remains valid and must continue to be supported.
• Coordinate command eligibility and fixed leading-argument counts with Align SPX event APIs with Auto Lambda fixed argument counts #3400, which tracks the necessary SPX event API alignment.

Acceptance criteria

• Applicable API-reference and completion snippets prefer the Auto Lambda spelling.
• Copilot can correctly recognize both spellings and prefers Auto Lambda in new code when the target command supports it.
• Guidance does not recommend Auto Lambda outside its supported classfile/command scope.
• Existing explicit-lambda user code remains valid and unchanged unless users choose to rewrite it.

[nighca]

#3400 (comment)

clone
onCloned { ... }

cloneWith T, data
onClonedWith T, data => { ... }

#3400 (comment)

broadcast "msg"
onMsg "msg" { ... }

broadcastWith "msg", T, data
onMsgWith "msg", T, data => { ... }

#3400 (comment)

onKey KeyXXX { ... }
onBackdrop "name" { ... }

onAnyKey key => { ... }
onAnyBackdrop name => > { ... }

[nighca]

The type-safe clone-payload API convention from the linked comment is now tracked in #3460. #3460 also aligns the XBuilder API Reference and completion treatment: prefer cloneWith / onClonedWith for payload-carrying clones, while this issue continues to govern Auto Lambda guidance for the no-payload form.