Create a strongly typed sandboxes map

[djeebus]

This gives us a place to put sandbox-specific code, to execute on insert, on delete, etc

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Note

Adds a sandbox-specific map with insert/remove hooks and execution-aware deletion, refactoring orchestrator components to use it.

• Core:
  • New sandbox.Map: Strongly-typed wrapper around smap with Subscribe, Insert, Remove, RemoveByExecutionID, Items, Get, Count, and NewSandboxesMap() in internal/sandbox/map.go.
  • Cleanup path now uses RemoveByExecutionID to avoid removing resumed sandboxes.
• API changes (type replacements):
  • *smap.Map[*sandbox.Sandbox] → *sandbox.Map across constructors and fields:
    • proxy.NewSandboxProxy, hyperloopserver.NewHyperloopServer/handlers.NewHyperloopStore.
    • metrics.NewSandboxObserver.
    • Server/service: internal/server structs and ServiceConfig.
    • Template build: build.Builder, layer.NewLayerExecutor, template server server.New.
    • Binaries/tests: main.go, cmd/build-template, benchmark_test.go, and sandboxes_test.go.
• Callsite updates:
  • Use sandbox.NewSandboxesMap() to construct.
  • Replace sandboxes.Insert(id, sbx) with sandboxes.Insert(sbx).
  • Update removal logic and proxy usages accordingly.

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[djeebus]

This prepares the codebase for #1333