refactor: reduce dependency count

[rkuester]

An audit in discussion #8 identified dependencies that can be removed or replaced with small amounts of custom code. Taken together, these changes could cut roughly half of our transitive crates.

Each removal is independent and makes a good self-contained PR. Pick a dependency from the audit, remove it (replacing with custom code if needed), run just checks, and open a PR. The dead dependencies are trivial and could all go in a single PR.

For removals that are debatable (e.g., replacing num-traits with manual IEEE 754 bit extraction), check in on the discussion first so we can agree it's worth doing before you write code.