[dancannon]

cueutil.LoadFromFS copies the whole CUE tree to a temporary directory for every service: thousands of file writes on a large app.

• Split it into WriteToDisk, which writes the tree once and returns a cleanup function, and LoadFromDir, which loads one service from it.
• computeConfigs first generates every service's CUE, then writes the tree once and loads each service from it.
• Add tests for loading several services from one tree, an incomplete config, and cleanup.

Why this should not change existing behaviour

• The load itself is unchanged. LoadFromDir contains the same CUE loading, orphaned-file handling and validation as the old function, with the temporary directory passed in.
• Each service still loads only the files under its own path. The old per-service tree held every service's files too, so the loader saw the same set of files per service. The only difference is that a service now sees the generated encore.gen.cue of every service in the tree, where before it saw those of the services generated before it. A config that imported another service's generated package would therefore have depended on service order, so none that works today can rely on that.
• On a large app the resulting config JSON for every service is identical before and after.
• LoadFromFS is internal to the encr.dev module and had no other callers.
• Errors still carry the temporary directory path, are built before cleanup runs, and the directory is removed on every path, including write failure. A write failure is reported on the first service that uses config, and a load failure on the service that failed, as before.
• Apps with no service config skip writing a tree altogether.

Testing

• go test -race ./pkg/cueutil/... ./v2/v2builder/...
• go vet, golangci-lint --new-from-rev=origin/main (0 issues) and a Windows build.

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