fix(log): send Debug and Trace output to stderr (#851)

[gustcol]

Context

Fixes #851

Root cause

Trace() and Debug() both called global.printf(..., os.Stdout).
When a user ran s5cmd --log debug cat s3://... and piped the output
into another program, diagnostic messages were interleaved with the
object payload, corrupting the stream.

Fix

Changed Trace and Debug to write to os.Stderr. Info continues
to write to os.Stdout (it carries functional output). Only Error
already used os.Stderr; this brings Debug and Trace in line with the
same convention.

Files changed: log/log.go

Tests

• Added TestDebugWritesToStderr — asserts Debug output appears on stderr and nothing on stdout.
• Added TestTraceWritesToStderr — same for Trace.
• Added TestInfoWritesToStdout — regression guard confirming Info still goes to stdout.

Manual verification

# Before: debug lines appear in stdout and break the pipe
s5cmd --log debug cat s3://bucket/object | wc -c   # inflated count

# After: only object bytes on stdout; debug lines on stderr
s5cmd --log debug cat s3://bucket/object 2>/dev/null | wc -c   # correct

[gustcol]

Test results

Unit tests (go test -race ./...): PASS (after adding e2e assertion fixes)

Changes included in this branch beyond the original fix:

• e2e/cp_test.go: 4 test functions updated — moved DEBUG assertions from result.Stdout() to result.Stderr(), asserted stdout empty
• e2e/pipe_test.go: 1 test function updated — same pattern
• e2e/sync_test.go: 6 test functions updated — for mixed-output tests (DEBUG + cp lines), split: DEBUG lines to result.Stderr(), cp lines remain in result.Stdout()

Wasabi smoke test:

$ ./s5cmd --log debug cp -n /tmp/smoke.txt s3://bucket/smoke.txt 2>/tmp/stderr >/tmp/stdout
$ cat /tmp/stdout     # empty
$ cat /tmp/stderr
DEBUG "cp /tmp/smoke.txt s3://bucket/smoke.txt": object already exists

Confirmed: DEBUG lines go to stderr; stdout stays clean.