[clintval]

Adds a README section, before Development and Testing, that finds the exons of a gene among per-target coverage rows with bedspec 4.3.0 and its new bedspec.ext.captainslog.
The example is the one in bedspec's README, and it runs under sybil like every other example, since bedspec ~=4.3 joins the test group.

Also in the README:

• Its examples run from tests/data, through a fixture in conftest.py, so paths read as a user's own files, like picard/hs.hs_metrics.
• Every file it names ends in .txt. The Picard fixtures are renamed to match, and scripts/fixtures/make.sh gives the outputs of the Picard tools that take only a prefix the .txt extension too, so regenerated fixtures keep these names.
• Each read stays on one line, except for the parenthesized with that opens two files.
• The variables holding a histogram or metrics are named for what they hold: isize_metrics, isize_histogram, and insert_size_histogram, in place of isize (used for two different things) and hist.
• The bedspec intro fits on one line.

bedspec joins typeline in the packages exempt from the one-week exclude-newer window, since both are released alongside captainslog.

uv run poe check-all passes: 1039 tests and 100% coverage.