test: raise coverage with edge-case tests, pextgen tests, and godoc examples

Add error-path and edge-case tests in bitset_test.go covering binary-order selection, FromWithLength/allocation/Compact edge cases, NextClear/PreviousClear boundaries, CopyFull, Equal length overflow, WriteTo/ReadFrom failures, UnmarshalJSON errors, Rank/Select out-of-range, shift limits, and Extract/Deposit/ExtractTo/DepositTo edge cases.
Add cmd/pextgen/main_test.go covering pextByte/pdepByte, their round-trip, generateTable, and main's flag handling and file generation.
Add TestIterEarlyStop to verify EachSet stops cleanly on an early break.
Add example_test.go and example_iter_test.go with self-verifying godoc examples for the common BitSet operations and the go1.23 EachSet iterator.
Raises core package coverage from 94.9% to 99.6% and cmd/pextgen from 0% to 94.2%.