Refactor leftovers: split RunState::start; move 33 pure e2e tests into the library

[JSBtechnologies]

Two small items left over from the file-split refactor (which shipped in 0.38.0). Neither changes behavior.

1. Split RunState::start into named steps

src/run_loop/setup.rs:11-288 is one linear function of about 280 lines, over half of them comments: config, terminal setup, the ctl bind, the initial spawn, the renderer, the safety net. Split it into named steps, the way run(), fleet_up, dispatch_ctl, build_request and spawn_pane_full were split. Each step becomes a function with a doc comment, and start becomes the sequence.

2. Move the pure e2e tests into library unit tests

33 tests in the end-to-end binary test library code with no pty and no e2e support:

module	tests	tests
tests/atrium/input.rs	16	atrium::input (the prefix scanner)
tests/atrium/bar.rs	10	atrium::bar
tests/atrium/filter.rs	6	atrium::filter
tests/atrium/resolve.rs	1	the command resolver

As unit tests in the library they run under cargo test --lib <module>, which is the only test command a fleet builder may run. They also stop compiling the whole e2e binary for a pure check. Move them as-is; the per-module test count must not change.

Done when

python dev.py check is green, the total test count is unchanged, and the Linux suite passes under WSL.