fix(sleep): read and write state.json as UTF-8

[PerryLink (PerryLink)]

skillopt_sleep/state.py reads and writes state.json with no explicit encoding, while save() deliberately passes ensure_ascii=False. The file therefore lands in the platform's locale charset, and two things follow -- the second is the serious one.

1. On a locale that cannot represent the text, save() raises. On a GBK machine:

UnicodeEncodeError: 'gbk' codec can't encode character '\u2705' in position ...

2. Even when it does not raise, the bytes are not UTF-8, and load() swallows the failure. load() wraps its decode in a bare except Exception: pass, so any reader whose locale codec is UTF-8 -- a container, CI, WSL, PYTHONUTF8=1, or simply another machine -- gets a decode error, discards it, and returns a fresh state:

locale.getencoding() = cp936
bytes on disk decode as UTF-8 : NO -> 'utf-8' codec can't decode byte 0xd6 in position 67
bytes on disk decode as GBK   : yes
as UTF-8  -> UnicodeDecodeError; load() swallows it via `except Exception: pass`
             -> a fresh state, i.e. every field silently reset

Night counter, harvest cursors, history and the cross-night memory all disappear without a warning. The file is readable only by the machine that wrote it.

This is the defect #124 fixed in skillopt/config.py -- "read YAML config files as UTF-8", merged 2026-07-12, one line:

-    with open(abs_path) as f:
+    with open(abs_path, encoding="utf-8") as f:

state.py was simply missed by that sweep. The change here is the same two lines:

-                with open(path) as f:
+                with open(path, encoding="utf-8") as f:
...
-        with open(tmp, "w") as f:
+        with open(tmp, "w", encoding="utf-8") as f:

Tests

Two tests in tests/test_sleep_state.py, and the second exists because the first is not sufficient:

• test_state_file_round_trips_non_ascii_as_utf8 -- saves a CJK lesson and a CJK project path, asserts the bytes on disk decode as UTF-8, and asserts the round-trip preserves both.
• test_state_io_does_not_fall_back_to_the_locale_codec -- repeats the round-trip in a subprocess under -X warn_default_encoding -W error::EncodingWarning.

The behavioural test cannot fail on a UTF-8 CI runner, because there the locale codec is UTF-8. On its own it would ship a regression test that never goes red. The warning-to-error guard makes CPython's own EncodingWarning a failure on every platform, which is what actually pins the fix.

Before (source fix reverted, tests kept):

FAILED tests/test_sleep_state.py::test_state_file_round_trips_non_ascii_as_utf8
FAILED tests/test_sleep_state.py::test_state_io_does_not_fall_back_to_the_locale_codec
EncodingWarning: 'encoding' argument not specified
2 failed, 1 passed

After:

3 passed

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