[fix](trx-config): keep the generated example off the machine that made it

The example is generated from the config defaults, and [decode_logs].dir
defaults to the running user's cache directory.  So the file rendered
/Users/sjg/Library/Caches/trx-rs/decoders on the machine that generated it and
/root/.cache/trx-rs/decoders in CI, and the up-to-date test failed for everyone
but its author.

Pin dir to an illustrative /var/lib/trx-rs/decoders in the example config;
omitting the key still falls back to the per-user directory.  A new test asserts
the rendered example contains none of this machine's home, cache or config
directories, so the next environment-derived default cannot slip through the
same way.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyX26FCpMQxiBoC7r5K1A7
Signed-off-by: Stan Grams <sjg@haxx.space>

trx-rs.toml.example

@@ -86,7 +86,7 @@ beacon_symbol_code = "-"
 # Write decodes to JSON Lines files.
 [trx-server.decode_logs]
 enabled = false
-dir = "/Users/sjg/Library/Caches/trx-rs/decoders"
+dir = "/var/lib/trx-rs/decoders"
 aprs_file = "TRXRS-APRS-%YYYY%-%MM%-%DD%.log"
 cw_file = "TRXRS-CW-%YYYY%-%MM%-%DD%.log"
 ft8_file = "TRXRS-FT8-%YYYY%-%MM%-%DD%.log"
@@ -95,6 +95,7 @@ wefax_file = "TRXRS-WEFAX-%YYYY%-%MM%-%DD%.log"
 
 # Which decoders run.  Trimming this list saves real CPU on small boxes.
 # Valid names: aprs, aprs_hf, ais, cw, ft2, ft4, ft8, lrpt, sstv, vdes, wefax, wspr.
+# output_dir sets where sstv/wefax/lrpt write images (default: user cache dir).
 [trx-server.decoders]
 enabled = [
     "aprs",