fix: eliminate -B double-dispatch through mock system

[Koan-Bot]

What

Fix the -B handler in _check_from_stat to avoid re-entering the mock system.

Why

The -B handler called -d $f_or_fh to check if the target is a directory. Since -d is also mocked under mock_all_from_stat, this triggered a full second round-trip through the mock dispatch — calling the user's stat callback again. This is wasteful and risks inconsistency if the callback has side effects or returns different results on subsequent calls.

Additionally, -B and -T were using lstat instead of stat, which is incorrect — both operators follow symlinks in Perl.

How

• Replace -d $f_or_fh with an inline mode-bit check (_S_IFMT mask against S_IFDIR) on the already-cached @stat array — zero mock re-entry.
• Add B and T to the $can_use_stat character class so the stat cache (not lstat) is used, matching Perl's symlink-following semantics.

Testing

• New t/B-no-double-dispatch.t verifies the stat callback is called exactly once per -B check (regular file, directory, non-existent file).
• Full test suite passes.

🤖 Generated with Claude Code

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Quality Report

Changes: 2 files changed, 63 insertions(+), 2 deletions(-)

Code scan: clean

Tests: passed (0 Tests)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[atoomic]

@Koan-Bot rebase

[Koan-Bot]

Rebase with requested adjustments

Branch koan.atoomic/fix-B-double-dispatch was rebased onto main and review feedback was applied.

Stats

2 files changed, 62 insertions(+), 1 deletion(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=None of the recent commits on main address the -B double-dispatch issue or the -B/-T lstat-vs-stat c)
• Resolved merge conflicts (1 round(s))
• Rebased koan.atoomic/fix-B-double-dispatch onto upstream/main
• Pre-push CI check: previous run passed
• Force-pushed koan.atoomic/fix-B-double-dispatch to origin
• CI check enqueued in ## CI (async)

CI status

CI will be checked asynchronously.

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