fix(memory): replace Unicode box-drawing chars with ASCII tree markers

[farizanjum]

Summary

Replaces Unicode box-drawing characters (└── U+2514, ── U+2500, ├── U+251C) in MemoryEntry.__str__() with ASCII tree markers (+--, |--). These characters caused UnicodeEncodeError crashes on Windows cp125x consoles.

Fixes #321

Changes

• mesa_llm/memory/memory.py: Replaced 7 occurrences of └──/├── with +--/|-- in MemoryEntry.__str__()
• tests/test_memory/test_memory_encoding.py: New regression test verifying output is encodable across 14 Windows code pages + UTF-8, and that no box-drawing characters (U+2500-U+257F) remain in output

Problem

MemoryEntry.__str__() used literal Unicode box-drawing characters as decorative tree markers in its output. When display=True (the default), Console().print(panel) writes these to sys.stdout. On Windows where sys.stdout.encoding is a cp125x variant (Western/Central/Eastern European, Cyrillic, Greek, Turkish, Hebrew, Arabic, Thai), these codepoints have no encoding mapping and raise UnicodeEncodeError, crashing the simulation.

This affects all memory backends (STMemory, STLTMemory, LTMemory) since they all call MemoryEntry.display().

Rich's safe_box correctly degrades its own panel borders to ASCII on legacy consoles, but the box-drawing characters in __str__() are literal content text -- Rich cannot degrade them, so they pass through to sys.stdout.write() and fail.

Solution

ASCII tree markers (+--, |--) are pure ASCII (bytes 0x2B, 0x2D, 0x7C). Safe in every encoding ever created. Preserves the tree hierarchy that └──/├── provided.

Options considered

Option	cp125x	cp437/850	cp932/936/949	Verdict
└── (current)	CRASH	safe	safe	broken
• (U+2022)	safe	CRASH	CRASH	trades one crash for another
+-- (ASCII)	safe	safe	safe	universally safe

Before / After

Before:

[Observation]
   └── price : $20
   └── seller : Bob
[Action]
   ├── (1)
   │  └── offer : $25
   ├── (2)

After:

[Observation]
   +-- price : $20
   +-- seller : Bob
[Action]
   |-- (1)
   |   +-- offer : $25
   |-- (2)

Testing

• All 121 existing memory tests pass
• 16 new encoding tests pass (14 parametrized code page tests + box-drawing absence + tree structure preservation)
• pre-commit run --all-files passes (ruff, ruff-format, pyupgrade, codespell)

python -m pytest tests/test_memory/ -v --timeout=30
# 137 passed

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