fix: hash the data in a parallel data file, not only the data we added - #2387

Fixes #2370.

Since 160ad3b (7.13.1) a parallel data file's name carries a hash, and combine skips files whose hash it has already seen. The hash only covered data added through that CoverageData object, so data that reached the file another way left the name describing different data. With coverage's own API alone:

cov = coverage.Coverage(data_suffix=True)
cov.start(); cov.stop()
cov.combine()   # pulls another parallel file's data in
cov.save()

On main the saved file is named with the empty-data hash (...HNQIp837rWOh) although it holds the combined data. In #2370, nbval has the notebook kernel write into pytest-cov's data file, which pytest-cov then saves with that empty hash. A python -c pass child gets the same hash, and combine skips and deletes the other file. Which one is lost depends on whether the host name sorts before H.

The fix: write() hashes the data that is in the file (the file, context, line_bits, arc and tracer tables in id order) and the per-call hashing in set_context and add_* goes away. Combine still reads the hash from the name, so 160ad3b's speedup stays.

Cost: one save got cheaper in my timings (arcs, 400 files x 250, 168 to 109 ms), but every write() now rescans the file, so a process that saves many times pays that each time. If that matters, the scan could run only when the file may have changed underneath us, for example by checking SQLite's PRAGMA data_version.

This removes the add_arcs hashing line that #2376 speeds up, so it overlaps that PR. #2376 measured the same arc cost. I'm happy to rebase onto it if it lands first.

Tests: two regression tests in tests/test_data.py (data written by another object, data from update()) fail on main and pass here. tox -e py312,lint,mypy passes, and the reporter's notebook repro (with a host name before H) goes from "No data to report" to 4 lines.

AI note: Claude Code prepared this on my behalf: it traced the cause, wrote the fix, tests and this text, and ran the checks. A separate Claude review pass checked it before I opened the PR.

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