Add timestamps to debug trace output

[officialasishkumar]

The --debug option writes magic-wormhole state-machine transitions to a stream. That stream now gets wrapped before being passed to debug_set_trace, so each line is prefixed with elapsed seconds from fowl startup.

This keeps the magic-wormhole trace API unchanged and makes the logs easier to time and compare between peers.

Changes:

• Add _TimestampedWriter, a small writable-stream wrapper that prefixes each new line with elapsed seconds.
• Use it for config.debug_file in create_fowl.
• Add a unit test for line-prefixing across partial writes.
• Add a NEWS entry.

Validation:

• python -m pytest --assert=plain --disable-warnings -q src/fowl/test/test_commands.py -k timestamped
• python -m pytest --assert=plain --disable-warnings -q src/fowl/test/test_commands.py
• python -m ruff check src/
• git diff --check

Note: I also started the full src/fowl/test unit command, but stopped it after it exceeded the 60 second CI timeout shape locally; terminating it caused Twisted subprocess teardown failures, so I did not count that as a validation run.

Fixes #111