replace package-level test doubles in ior.go with constructor injection (DIP)

Introduce runnerDeps struct to bundle all injectable function dependencies
(getEUID, runTrace, runParquet, runTraceWithContext, runTUI*) that were
previously package-level vars overridden in tests. The modeRegistry now
carries a runnerDeps instance and passes it to each handler's run() method,
eliminating global state mutation in tests.

- Add runnerDeps struct and defaultRunnerDeps() constructor in ior_mode_registry.go
- Convert modeRegistry from a []modeHandler slice type to a struct with
  handlers + deps fields; add newModeRegistry(deps) constructor
- Update modeHandler.run() signature to accept runnerDeps; handlers call
  deps.getEUID / deps.runTrace etc. instead of globals
- Update SetTUIRunners to write into defaultRegistry.deps instead of
  package-level vars
- Add dispatchRunWithDeps helper for test isolation without global mutation
- Remove root-privilege check from runTraceWithContext and runHeadlessParquet;
  each mode handler owns the EUID gate via deps.getEUID
- Rewrite ior_mode_test.go: replace save/restore patterns with stubDeps()
  helper and dispatchRunWithDeps; add three new root-privilege tests
  replacing the removed TestRunTraceWithContextRequiresRoot

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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