Fence gates turn farmland and dirt paths into dirt

Summary

Placing a fence gate above farmland or a dirt path converts the block below into dirt. Vanilla explicitly exempts fence gates from this obstruction rule.

Current behavior

Both support checks use only the legacy-solid flag of the block above:

• crates/pumpkin/src/block/blocks/farmland.rs::can_place_at
• crates/pumpkin/src/block/blocks/dirt_path.rs::can_place_at

let state = world.get_block_state(&block_pos.up());
!state.is_solid() // TODO: add fence gate block

Generated fence-gate states have the legacy IS_SOLID flag, so the checks fail and schedule conversion to dirt. Both files already carry the explicit fence-gate TODO.

Vanilla behavior

Vanilla Java keeps farmland and dirt paths intact under fence gates. Its obstruction rule treats legacy-solid blocks as blockers with a fence-gate exception; farmland also has a moving-piston exception.

References:

• https://minecraft.wiki/w/Block_properties/Solid_(legacy)
• https://mojira.dev/MC-307716

Reproduction

1. Place farmland and a dirt path.
2. Put any fence gate directly above each block.
3. Wait for the scheduled block update.

Pumpkin converts both blocks to dirt. Vanilla keeps both unchanged.

Suggested fix

Update both can_place_at helpers to allow fence gates despite their legacy-solid flag. Preserve the remaining vanilla exceptions, including moving pistons for farmland, and add tests for closed/open fence gates plus an ordinary full solid block that should still cause conversion.

Verified against current master at 1ac447f2918b7dec7d19a5836fa3673d4fe3c9f5; the connected issue list and targeted repository search found no existing farmland/dirt-path fence-gate issue.