fix: correct -e false negative for stat results with mode 0

[Koan-Bot]

What

Fix -e returning false for mocked files whose stat has mode 0.

Why

The -e handler in _check_from_stat tested scalar @stat && $stat[ST_MODE].
Since mode 0 is falsy in Perl, any file mocked with all-zero stat fields
(or just mode 0) would incorrectly report as non-existent. In Unix semantics,
stat() success alone implies existence — mode is irrelevant for that check.

Also fixes a gap in the XS stat array size validation where a 1-element array
(AvFILL == 0) slipped through the > 0 guard undetected.

How

• Changed -e handler to _to_bool(scalar @stat) — existence depends only on stat success
• Changed XS validation from av_size > 0 to av_size >= 0 with improved error message
• Updated test expectation: [(0) x 13] means "file exists with all-zero fields"

Testing

Full test suite passes (make test).

🤖 Generated with Claude Code

[atoomic]

@Koan-Bot rebase

[Koan-Bot]

Rebase with requested adjustments

Branch koan.atoomic/fix-e-mode-zero-false-negative was rebased onto main and review feedback was applied.

Stats

3 files changed, 5 insertions(+), 5 deletions(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=The -e handler on main still tests $stat[ST_MODE] which is falsy for mode 0, and the XS guard still )
• Resolved merge conflicts (1 round(s))
• Rebased koan.atoomic/fix-e-mode-zero-false-negative onto upstream/main
• Pre-push CI check: previous run passed
• Force-pushed koan.atoomic/fix-e-mode-zero-false-negative to origin
• CI check enqueued in ## CI (async)

CI status

CI will be checked asynchronously.

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