chore(cargo, justfile): Stop building empty test binaries

[JeanMertz]

just test builds and lists 52 test binaries instead of 74, running the same 6444 tests. Every binary that went away held no tests at all, and nextest runs each one it builds to enumerate its contents, so an empty binary cost a link and a process spawn for nothing.

They were there because --all-targets overrides the test flag a manifest sets (rust-lang/cargo#8338), so every executable was built as a test harness regardless, and the six libraries already carrying [lib] test = false had their opt-out ignored along with them. The test recipes pass no target selection now, which is the only mode that honours the flag and still picks up new crates, new integration tests, and examples as they appear.

All 13 binaries in the workspace now set test = false, which meant moving jp-gui, jp-serve-web, and jp-ticket into libraries with their executables reduced to a shell, so the 103 tests that lived in those binary crates keep running. Two checks catch a test that would otherwise silently not run: lint-ci rejects a #[cfg(test)] inside a binary that excludes itself from testing, and
cargo shear --check-test-targets covers libraries in both directions, now that shear-ci denies warnings. Denying them also turned up an optional dependency schematic declared and never used, now removed.

The nextest profile builds with line tables rather than full debug info. Backtraces keep their file and line numbers; a debugger attached to a test binary loses its locals. Override the profile when stepping through one.