[feat] make preview_hard_zoom.png a hard requirement (backfill main + the in-flight PRs)

[BenchCAD]

Follow-up to #18, which lands preview_hard_zoom.png (front/side/top/iso of a hard example) as a bench2 preview output but only warns when a family PR is missing it.

Reason for warning first: PRs branched before #18 cannot generate the file, and main itself is not yet conformant. Hard-failing on day one would have turned 11 open community PRs red at once.

To flip it to required

1. Backfill main — re-run uv run bench2 preview <family> and commit preview_hard_zoom.png for the families merged without it: slotted_din_rail, speaker_pole_mount_socket (one PR per family, per the one-family rule).
2. Drain / nudge the in-flight family PRs — as of the [framework] bench2 preview emits preview_hard_zoom (three-view+iso) + CI gate (warns first) #18 merge these carry it: [family] articulating_monitor_arm_vesa_plate: VESA FDMI adapter plate #11, [family] round_duct_butterfly_damper: round HVAC balancing damper #12, [family] drum_tension_lug: drum-shell tension lug #15, [family] guitar_tuning_machine_head: sealed geared tuning machine #22, [family] tilting_speaker_wall_bracket: tilt-adjustable speaker wall mount #23. These do not (rebase on main, re-run bench2 preview, commit the fourth render): [family] minifix_cam_housing: Häfele Minifix 15 connector housing #34, [family] set_screw_shaft_collar: GN 705 / DIN 705 set collar #36, [family-assembly] cabinet_bar_pull_handle: cabinet/drawer bar pull #42, [family] duct_takeoff_starting_collar: round-duct start collar #43, [family] single_row_deep_groove_ball_bearing: ISO 15 6000-series open bearing #44, [family-assembly] truss_half_coupler_hook: rigging half coupler / hook clamp #47, [family] quarter_turn_cam_latch: Southco E5-class quarter-turn cam latch #48, [family] vertical_latch_toggle_clamp: GN 851.1 vertical latch toggle clamp #50, [family] metric plastic cable gland #52, [family-assembly] Add t_handle_ball_lock_pin #54, [family-assembly] window_cam_lock: AmesburyTruth 17 Series cam lock #56. CI already tells each of them exactly this in a warning annotation.
3. Flip the gate — in .github/workflows/family-pr-checks.yml, move preview_hard_zoom.png from step 3b (warn) into step 3's required-files loop, restore the "seven files" wording in the step-3 error message, the header comment, CONTRIBUTING.md's gate table, and drop the "warns while … backfill" caveat.

Steps 1 and 2 can happen in any order; step 3 only after both are done, so no PR goes red for something its author could not have produced.