fix(profiler/behave_shell): tolerate non-UTF-8 bytes in shard reads

Real-world bug surfaced on the first live decky run: sessrec.c's
json_escape (decnet/templates/_shared/sessrec/sessrec.c:111-141)
only escapes bytes < 0x20 + DEL — bytes >= 0x80 pass through raw.
An attacker pasting Latin-1 / GB18030 / any non-UTF-8 8-bit text
yields a shard line that chokes Python's default UTF-8 text-mode
read with 'utf-8 codec can't decode byte 0xac'.

Three changes:

1. _events_for_sid now opens with errors='surrogateescape', preserving
   byte fidelity through the JSON parse. Surrogate-half chars
   correctly fail isascii() / isalpha() so the typed-letter
   histograms filter them out automatically. Tightening sessrec.c to
   escape >= 0x80 is filed for v0.2 — that's the proper forensic-data
   fix; the surrogateescape read makes the engine robust meanwhile.

2. Regression test
   (test_handler_tolerates_non_utf8_bytes_in_shard) builds a shard
   with raw 0xAC bytes inside a JSON 'data' string and asserts the
   handler still persists observations.

3. Collector's _emit_session now logs at WARNING (was DEBUG) when
   find_shard_with_sid returns None, citing the three usual causes
   (ARTIFACTS_ROOT perms, _SERVICE_RE whitelist, sessrec/collector
   race). Surfaces the silent-skip class of bug in seconds instead of
   hours — the first live run hid a perm mismatch
   (User=anti without SupplementaryGroups=decnet) for an entire
   session window before the symptom was traced upstream.

decnet/profiler/behave_shell/_handler.py

@@ -53,9 +53,17 @@ def _events_for_sid(shard: Path, sid: str) -> list[AsciinemaEvent]:
     Mirrors the loader pattern in
     ``tests/profiler/behave_shell/test_calibration_grid.py``: skip
     headers / non-matching sids / unparseable lines silently.
+
+    ``errors="surrogateescape"`` because sessrec.c's json_escape only
+    escapes bytes < 0x20 + DEL — bytes >= 0x80 pass through raw, so
+    a real shard with Latin-1 / GB18030 / arbitrary 8-bit attacker
+    paste content is NOT valid UTF-8. surrogateescape preserves byte
+    fidelity through the JSON read; downstream isalpha() / isascii()
+    correctly filter the surrogate-half chars out of the typed-letter
+    histograms. Filed for v0.2: tighten sessrec.c to escape >= 0x80.
     """
     events: list[AsciinemaEvent] = []
-    with shard.open() as f:
+    with shard.open(encoding="utf-8", errors="surrogateescape") as f:
         for line in f:
             try:
                 rec = json.loads(line)