No make target switches the stack into the e2e posture Playwright needs

[catinspace-au]

The dfe-ui Playwright suite needs the engine running with DFE_E2E_SERVER set and DFE_ENV=test. dfe-docker ships no switch for that, so every run means hand-editing .env, and remembering to put it back. #143 carries an e2e-posture target, but inside a large branch that has conflicts with main.

• Lift the posture switch out of test(e2e): kill hops under load and count the loss #143 into its own small change: one target turns it on, one restores the previous .env.
• The Playwright run should check the posture first and refuse with a clear message, not fail forty specs in.

Done when a fresh checkout runs the Playwright suite with one make target and goes back to normal with another.