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fix(codegen): mask stale generated outputs from the loader instead of unlinking them

Root cause of the check-generate CI failures on this PR, found by running
`go generate ./...` locally: base and master pass, every commit of this
branch failed deterministically (the "different type each run" in CI was
map-iteration order picking which missing type to report first — the
failure itself was 100% reproducible).

The old upfront `syscall.Unlink` of the exec + model outputs was not
redundant — it was LOAD-BEARING for every config that autobinds the
package its models are generated into (all codegen/testserver fixtures,
the federation examples):

  1. old: unlink first → stale models file GONE at schema-load time →
     autobind finds nothing → modelgen generates all types → OK.
  2. this branch (before this commit): stale models file VISIBLE at
     load → autobind binds the previously-generated types as if they
     were user-written models → modelgen SKIPS them → the freshly
     written models file loses them → the exec build reload fails with
     "unable to find type: <pkg>.<Type>".

But the unlink is also exactly what #2345/#3505 are about: delete-then-
interrupted leaves the user with no generated file at all. Both
properties need to hold, so replace the destructive unlink with a
NON-destructive loader mask:

  - internal/code.Packages gains an Overlay (packages.Config.Overlay)
    threaded into every Load, plus MaskFile/UnmaskFile.
  - api.generate masks each existing output (exec + model) with a
    package-clause-only stub read from the file's own package clause
    (empty bytes would be a parse error and break loading the rest of
    the package). Missing/unparseable files are left alone — nothing
    stale to bind, matching the old unlink's no-op there.
  - config.Config owns the overlay map (MaskGeneratedFile) because
    LoadSchema RECREATES c.Packages mid-generation — an overlay set
    only on the first instance would silently vanish. The ... (continued)