fix: stdbuf -oL on the tr pipe (1.0.0-58)

1.0.0-57's tr-pipe fix delivered \n-terminated progress lines but
tr's stdout is block-buffered (4 KB chunks) when its destination
is a pipe — and the SSH channel is a pipe. At ~50 bytes per badblocks
progress line, that means ~80 lines accumulate (~6 minutes at our
throughput) before tr flushes anything.

stdbuf -oL forces tr's stdout to line-buffered mode. Each \n now
triggers a flush. Progress lines reach asyncssh as they happen.

app/burnin/stages.py

@@ -124,11 +124,11 @@ def _build_badblocks_cmd(devname: str) -> str:
         f"-p {settings.surface_validate_passes} "
         f"/dev/{devname} 2>&1"
     )
-    # The outer pipeline lets tr translate \\b → \\n. Single quotes
-    # around the inner script prevent the host shell from interpreting
-    # $$. The tr argument uses $'\\b' / $'\\n' escapes — POSIX shells
-    # interpret these as the literal backspace and newline bytes.
-    return f"sh -c '{inner}' | tr '\\b' '\\n'"
+    # The outer pipeline lets tr translate \\b → \\n. stdbuf -oL forces
+    # tr's stdout to line-buffered mode; without it tr's stdout is
+    # block-buffered (4 KB chunks) when its destination is a pipe,
+    # which delays each progress line by ~6 minutes at our throughput.
+    return f"sh -c '{inner}' | stdbuf -oL tr '\\b' '\\n'"
 
 from . import kill
 from ._common import (

app/config.py

@@ -86,7 +86,7 @@ class Settings(BaseSettings):
     ssh_key: str = ""             # PEM private key content (paste full key including headers)
 
     # Application version — used by the /api/v1/updates/check endpoint
-    app_version: str = "1.0.0-57"
+    app_version: str = "1.0.0-58"
 
     # ---- Authentication (1.0.0-22) ----
     # session_secret: HMAC key for signing session cookies. Empty = generate