feat: add stat_as_fifo helper for named pipe mocking

[Koan-Bot]

What

Adds stat_as_fifo() helper — the only file type missing from the stat_as_* family.

Why

Every file type with a -X check operator has a corresponding stat_as_* helper (directory, file, symlink, socket, chr, block) except FIFO/named pipe. Users of mock_all_from_stat who need to simulate a FIFO had to manually construct the stat array with the correct S_IFIFO mode bits, while every other type has a one-liner helper.

How

• Added stat_as_fifo() following the exact same pattern as the existing helpers
• Added to @STAT_HELPERS, exports, and POD
• Added unit test in stat-helpers.t and integration test in mock-all-from-stat_basic.t (verifies -p, -e, !-f, !-d, !-S behavior)
• Fixed CLAUDE.md referencing a non-existent mock_lstat function

Testing

Full test suite passes (make test — all tests green).

🤖 Generated with Claude Code

[atoomic]

@Koan-Bot rebase

[Koan-Bot]

Rebase with requested adjustments

Branch koan.atoomic/add-stat-as-fifo-helper was rebased onto main and review feedback was applied.

Stats

4 files changed, 25 insertions(+), 2 deletions(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=None of the recent commits on main mention stat_as_fifo, S_IFIFO, FIFO, or named pipe — the feature )
• Resolved merge conflicts (1 round(s))
• Rebased koan.atoomic/add-stat-as-fifo-helper onto upstream/main
• Pre-push CI check: previous run passed
• Force-pushed koan.atoomic/add-stat-as-fifo-helper to origin
• CI check enqueued in ## CI (async)

CI status

CI will be checked asynchronously.

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