overlay/replace on a map replaces keys, not only values

[kralikba]

What steps did you take:
I tried applying overlay/replace to all child elements' values.

a:
 a1:
   a: 1
   b: 2

#@ load("@ytt:overlay", "overlay")

#@overlay/match by=overlay.all
---
a:
#@overlay/match by=overlay.all, expects="1+"
#@overlay/replace
 _:
   a: 1
   b: 3

What happened:
Not only the values, but their key was replaced as well (to the dummy placeholder value).

a:
  _:
    a: 1
    b: 3

Because of this, matching overlay.all won't really work, as for multiple child items, this will cause a duplicate key error.

What did you expect:
That only the values are replaced

a:
  a1:
    a: 1
    b: 3

Anything else you would like to add:

Using overlay.map_key instead of overlay.all results in the same erroneous behaviour.
overlay/merge behaves as expected (does not replace key) with both matchers.

Environment:

• ytt version (use ytt --version): 0.51.1
• OS (e.g. from /etc/os-release): Windows 11 24H2 26100.3194

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