IR-136: a broker bound under the seed socket's name is not seed control. This adds a shared, test-only `ServingFixture` (crates/spt-daemon/tests/support/serving.rs) that composes canonical seed control + broker + docs listener, and migrates the docs-server e2e tests and twohost B onto it. Twohost A is unchanged.

Gate (doyle): test-only diff (3 test files + register + toml), parent 52d56248 (IR-135, now main). `traceable-reqs check` 0.4.1 exit 0 in an isolated worktree. Windows integration compile verified on hfenduleam: `cargo check --tests -p spt-daemon` exit 0 at 84e20ddd (thin ci's unit lane does not compile integration tests). hertz: Linux clippy 0, 3 real docs cases green, 7 unopted twohost passes are NOT cross-host proof; migrated twohost B gets its cross-host proof on the next golden run.

Co-authored by: hertz

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