Add the interview engine, follow-up angles, prompts and guard
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Commit

- Plain-code engine: small talk, questions, one follow-up each, reactions, candidate questions, closing; answers collected across pauses until "I'm done" or 5 s of quiet; the package always gets '' and the engine speaks its own lines
- Follow-up angle chosen by rules (short, we vs I, no outcome, missing key point, edge case, motivation, deeper); Gemma words it with the run 3 prompt, a 6 s limit, then a written line for that angle
- Guard: question-word lines ending in '.' and instructions accepted, reactions that are new questions rejected; on the recorded run 4/5 lines it passes 8/12 for Gemma 3 1B and 12/12 for Gemma 4 E2B
- Session wires the engine to the voice, Gemma and the interview screen; I'm done, Repeat, Skip and End work
- System design round parked with an agreed scope (PLAN.md)
Entire-Checkpoint: 01M41Q1MACBZQ93ZGYQ85J1DEF
Checkpoints
Checkpoint 1