feat: add mock_file_checks_guard() for multi-check scoped mocking

[Koan-Bot]

What

Adds mock_file_checks_guard() — mock multiple specific file checks at once, get one guard back.

Why

Current API has a gap between mock_file_check_guard (single check) and mock_all_file_checks (all checks). Tests that need 2–5 specific checks mocked together require managing multiple guard objects or manual cleanup. This fills that gap with a single call.

How

Pure Perl convenience wrapper: iterates check/CODE pairs, calls mock_file_check for each, returns one Guard object covering all checks. No XS changes.

my $guard = mock_file_checks_guard(
    '-e' => sub { CHECK_IS_TRUE },
    '-f' => sub { CHECK_IS_TRUE },
    '-d' => sub { CHECK_IS_FALSE },
);
# all three unmocked when $guard goes out of scope

Testing

16 new tests covering: basic multi-check setup/teardown, FALLBACK_TO_REAL_OP passthrough, guard cancel, error cases (odd args, empty args, duplicate mock), mixed dash/no-dash syntax. Full test suite passes.

🤖 Generated with Claude Code

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Quality Report

Changes: 2 files changed, 151 insertions(+), 1 deletion(-)

Code scan: clean

Tests: passed (0 Tests)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline