Stop rigid boat hull breaking into pliable fabric sheets

[cake-pie]

Summary

Bugfixes "Stop rigid boat hull breaking into pliable fabric sheets"

Purpose of change

The vehicle part carbonfiber_boat_hull built using item of same id is of material kevlar_rigid but breaks into whole sheets of pliable kevlar fabric sheet_kevlar.

Cataclysm-DDA/data/json/vehicleparts/vehicle_parts.json

Line 1132 in 583923e

"breaks_into": [ { "item": "sheet_kevlar", "count": [ 1, 3 ] } ],

Such rigid kevlar material is made from multiple layers that have been bonded (with epoxy resin IRL; superglue in-game) and should not delaminate into intact sheets.

Describe the solution

Change outcome to rigid Kevlar plate rigid_kevlar_plate which corresponds to material kevlar_rigid.

Add word "rigid" to the item description (compare: plastic_boat_hull is "A rigid plastic sheet that keeps the boat afloat."); this is to emphasize that it is of rigid construction, not "skin-on-frame".

Describe alternatives you've considered

• More damage, smaller pieces: requires creating a new item that smaller sized version of rigid_kevlar_plate.
• Total obliteration: instead of rigid plates or intact sheets, only scraps of Kevlar fabric scrap_kevlar remain.

Testing

Checked valid json.
Unfortunately, smashing a racing kayak to bits with a sledgehammer in game tends to break the frame before the hull part.

Additional context

This PR does not attempt to fix the mismatch in weight of maximum breaks_into output vs the part itself, which dates back a long way. carbonfiber_boat_hull has weighed 500g ever since it was added, but the breaks_into output has varied, and most probably has never come anywhere close to matching the item weight:

• up to 3 × kevlar_plate @ 360g each when added in vehicles: add support for amphibious vehicles #27251
• up to 3 × kevlar_plate @ 300g each just before Rework kevlar, add slash resistant and EOD gear #39964
• up to 3 × sheet_kevlar @ 5g each just after Rework kevlar, add slash resistant and EOD gear #39964
• up to 3 × sheet_kevlar @ 26g each as of this writing (583923e)
• up to 3 × rigid_kevlar_plate @ 429g each after this PR