Fix #827: write color tag on bed block entities

[coopdogGGs]

Issue: #827 - beds generate incorrectly.

Root cause: bed block entities were written without the color tag, so Minecraft rendered them incorrectly.

Fix: set_bed_block_entity_absolute now writes color on the bed block entity.

Tests added: bed_block_entity_includes_red_color_tag.

Verification: full cargo test suite 361 passed, 0 failed.