Enable "plain merge" for overlays, in general

With:
- #381
- #418

... there is now a general mechanism for accepting a plain YAML file and converting it into an overlay with well-defined semantics:

1. new keys are added (like `@overlay/match missing_ok=True`)
2. existing values are replaced (like `@overlay/replace or_add=True`)

A working name for this overlay strategy is, "plain merge."

**Describe the solution you'd like**
Users may encounter other use-cases where performing this kind of overlay would be useful.

Provide a means for allowing the user to specify this strategy wherever an overlay operation occurs:

1. `@data/values plain=True`
2. `@data/values-schema plain=True`
3. `@overlay/match plain=True` (especially valuable when annotating a document)
4. `overlay.apply(..., plain=True)`

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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start by tracing the existing overlay operations and the four entry points named in the issue: @data/values, @data/values-schema, @overlay/match, and overlay.apply(...). Determine how plain merge should be represented consistently across annotation and API forms, then verify that new keys are added and existing values are replaced in each supported path.

Written by the indexing model from the issue text.