fix: wrap _assign() method calls in eval with hash fallback

[toddr-bot]

What

Add eval wrapping and hash-access fallback to _assign() for blessed object method calls.

Why

The _assign() method called $root->$item(...) on blessed objects without an eval wrapper — a long-standing TODO noted in the commented-out code at lines 622-637. When the object is a blessed hashref and the method doesn't exist, the assignment silently failed instead of falling back to hash key access.

This is inconsistent with:

• _dotop() (line 496) which wraps method calls in eval and falls back to hash/array ops
• The XS stash assign() which checks method existence via gv_fetchmethod_autoload before calling

How

• Wrap both the default-check and assignment method calls in eval { }
• Propagate real errors (exception objects or methods that exist but throw)
• Fall back to hash key assignment for hash-based objects when method is missing
• Mirrors the same pattern used in _dotop() for consistency

Builds on PR #384 (the isa() fix for $atroot detection).

Testing

• Added 3 tests with a PartialObject (blessed hashref with some methods): read method, read hash key, assign to hash key via fallback
• Verified old code silently drops the assignment; new code correctly assigns via hash access
• Full test suite passes (all 116 test files)

🤖 Generated with Claude Code

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Quality Report

Changes: 2 files changed, 92 insertions(+), 20 deletions(-)

Code scan: clean

Tests: passed (OK)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[toddr-bot]

Recreated from #385 (auto-closed when the toddr-bot fork was removed). Original branch is now hosted on this repo.