Avoid contradictory shield material names

[henriquejsza]

Closes #232.

Summary

• rename the ordinary small shield's true name to buckler
• use small shield as the unidentified description for all three matching shield types
• update the Valkyrie level reference and tile metadata to the new names

This keeps randomized material names coherent: an unidentified variant is now a stone small shield, while its identified ordinary form is a stone buckler.

Testing

• make all
• make install
• focused wizard-mode Lua regression: reproduced the old three wooden shield definitions before the change, then passed with ISSUE_232_REPRO_OK
• all-object creation loop from test/test_obj.lua: passed with OBJECT_ENUM_OK

The later interactive item-use section of test/test_obj.lua was not used as an automated gate; the repository CI itself builds on Linux, macOS, and Windows.