tests: isolated scenario suite (parity with the monolith, fast --only)

Replace the 1200-line monolithic run() — one giant pilot session whose
100 checks shared mutable app state, so a single break cascaded and every
edit meant re-running the whole ~49s suite — with independent @scenario
functions over one shared boot.

Each scenario re-establishes its own function context and is wrapped so a
crash (or a check failure) fails that scenario alone instead of aborting
the rest. A reset() baselines between scenarios: dismiss modals, hide the
names pane and input widgets, clear the filter, default _pref=decomp.

A Ctx helper wraps app+pilot (check/wait/press/open/goto_ui, biggest(),
pick_decomp_ref(), ...). CLI: --only <substr,...> runs a subset in a few
seconds (structs in ~1.5s vs 49s), --stop-after, --list. Per-scenario
timing is printed, which is what surfaced a decomp-default view-flip that
had been passing a jump+back check trivially.

Full run: 101 checks green in ~41s.

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