Make modtools/create-unit Steam-arena compatible and add spawn-unit delegator

[unboundlopez]

Motivation

• Prevent hard crashes when modtools/create-unit is invoked on Steam DFHack builds that expose arena state under a different field layout.
• Make arena-related setup/restore resilient to missing optional fields so spawning can proceed or fail with a clear diagnostic instead of a nil-index error.
• Provide a small, prompt-driven spawn-unit wrapper that preserves interactive flow while delegating actual unit creation to the Steam-aware modtools/create-unit implementation.

Description

• Update modtools/create-unit.lua to prefer df.global.world.arena_spawn and fall back to df.global.world.arena, and qerror if no arena spawn state is available.
• Guard optional arena fields (type, filter, interaction, tame) with nil checks and only restore them if previously saved, preventing nil-index crashes on Steam layouts.
• Make arena-open and selection input resilient by attempting ARENA_CREATE_CREATURE first then D_LOOK_ARENA_CREATURE, and use a table-form SELECT fallback when direct SELECT delivery fails.
• Add a new spawn-unit.lua script that provides an interactive list/input prompt flow, validates fortress mode and cursor position, validates race/caste and count, and delegates spawning to create_unit.createUnit(...) inside a pcall, exposing do_spawn when loaded as a module (dfhack_flags.module).

Testing

• Static pattern checks using rg confirmed the arena_spawn fallback and the key/input fallbacks are present in modtools/create-unit.lua (succeeded).
• File inspection with nl/sed verified the guarded field handling and the new spawn-flow code paths are present (succeeded).
• No end-to-end Steam DF/DFHack runtime was available in this environment, so no live spawn integration could be executed (not executed).

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fail loudly when arena menu cannot be opened

In createUnitInner, if neither arena-open key can be injected, the code just breaks out of the spawn loop and returns whatever was created so far (often zero) without an error. That turns an environment/keybinding incompatibility into a silent under-spawn, which is especially misleading for callers like spawn-unit that treat a returned table as success. This path should raise a diagnostic instead of silently exiting.

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[chatgpt-codex-connector]

Stop processing when creature SELECT input fails

If SELECT fails, the fallback pcall result is ignored and execution continues as if a creature was spawned. In that failure case, the subsequent df.unit.find(df.global.unit_next_id-1) can point to a pre-existing unit, so processNewUnit(...) may mutate the wrong creature (nickname/domestication/flags), corrupting game state. The spawn path should verify that selection succeeded (or that a new unit id was allocated) before continuing.

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