fix: fail check-coverage when there is no coverage data to check

[haigou-web]

Fixes #610

Problem

With no coverage data at all, --check-coverage exits 0:

c8 --check-coverage --lines 100 node -e "1"   # exits 0, should exit 1

Two separate gaps:

1. istanbul reports pct as the string "Unknown" when there is no data, and "Unknown" < 100 is false — the threshold check silently passes.
2. With --per-file, map.files() is empty, so the forEach never runs and nothing is checked at all.

Fix

• Treat "Unknown" as failing any threshold above 0
• With --per-file, fall back to the overall summary when there are no files
• A threshold of 0 still passes (it does not require coverage)

Note on an existing test

exits with 0 if coverage within threshold used --exclude="test/fixtures/*.js", which excluded the only file the before hook had instrumented. That test was relying on the bug: it was asserting "no data → exit 0" while its name says "coverage within threshold → exit 0". I changed the exclude to test/fixtures/all/** so the test actually exercises its stated intent.

Verification

• New tests: no data (exit 1), no data + --per-file (exit 1), no data + threshold 0 (exit 0)
• Full suite: 99 passing/14 failing before → 105 passing/14 failing after; zero new failures, zero disappeared
• The 14 pre-existing failures are monocart / --extension related and fail identically on an unmodified checkout