fix(#392): keep the '[seeded] planted answer' marker off the board

[duckyquang]

Fixes #392.

Summary

A seeded turn with no supplied utterance got a synthetic [seeded] planted answer: 'X' content default. That was harmless until #377 started rendering Turn.content under --show-rationale -- now the holdout reads the harness's own "this is planted" tag, and any referee/judge scanning whole turns (referee._default_shortcut_detector, blind_metric.latch_rate) treats the marker as stated reasoning. Cannot-fail-by-construction, #374 family.

Dropped the marker: the seeded flag and the committed answer already record the plant, so no supplied content means empty content. anchored_seed/prescreen_flag already pass real prose for exactly this reason.

Notes

No committed number is affected -- nothing's been run with --show-rationale yet -- but it's armed for the next #270/#271/#115/#221 run. Pinned two ways: the seeded turn's content stays empty, and render_board(show_rationale=True) doesn't surface the marker. Both went red on the marker before the fix.

The two adjacent renderer issues you flagged in the same pass (multi_round.py indent, the tripwire regex) are split out into #412 rather than bundled here, since they're coupled to each other and separate from the marker leak. @sebasmos mind a look?

[sebasmos]

Verified: the marker is off Turn.content, board render confirmed clean under show_rationale=True, suite at 1159 passed. Sabotage-checked by reintroducing the old [seeded] planted answer string, both new tests go red exactly as expected. Clean fix, approving.