feat(internal): stdout transport

[jpnurmi]

The inproc-based Android NDK uses SENTRY_TRANSPORT=none and installs a JNI function transport that writes to outbox/, leaving HTTP delivery to Java. Tests substitute it with a stdout callback to inspect envelopes without Java or HTTP.

However, the Native daemon in #1725 cannot use the function transport from the separate daemon process. Provide (an internal) built-in stdout transport (SENTRY_TRANSPORT=stdout) that both the SDK and daemon can instantiate. This allows us to run test_integration_stdout.py for Native/Android in #1725.

The undocumented stdout transport is mostly intended for our own integration testing purposes, but it's also handy for quick experiments:

$ make build SENTRY_BACKEND=inproc SENTRY_TRANSPORT=stdout
...
$ ./build/sentry_example_crash 2>/dev/null
{"event_id":"0e1d5355-5ba8-42cb-9e96-c6610d4ee9c1",...}
{"type":"event","length":2823}
{"event_id":"0e1d5355-5ba8-42cb-9e96-c6610d4ee9c1","timestamp":"2026-10-16T16:19:26.968633Z","platform":"native","level":"fatal","exception":...}
...

#skip-changelog (internal)

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.16%. Comparing base (4ce28e7) to head (c5068b8).

Additional details and impacted files

@@            Coverage Diff             @@
##               v1    #2213      +/-   ##
==========================================
+ Coverage   75.13%   75.16%   +0.02%     
==========================================
  Files         104      105       +1     
  Lines       29630    29648      +18     
  Branches     5479     5480       +1     
==========================================
+ Hits        22263    22284      +21     
+ Misses       5960     5957       -3     
  Partials     1407     1407              

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