Proposal 0015: bb_lua

[jimsynz]

Summary

Adds proposal 0015 — bb_lua, a sandboxed Lua scripting layer for Beam Bots robots, mirroring ash_lua's thin-layer-over-{:lua, "~> 0.3"} design.

Key shape:

• BB.Lua.Robot Spark extension with a lua do … end block (expose_commands, expose_topics, expose_parameters) — explicit opt-in surface
• Go-style (result, err) return shape with a closed error code set aligned to BB.Command.await/2
• Host-supplied safety/identity context that is not readable from Lua; scripts cannot arm/disarm
• BB.Lua.PubSubBridge for bb.on(name, fn) event subscriptions, with per-topic throttling for high-frequency topics (joint state at 100 Hz)
• Marshalling table for Vec3 / Quaternion / Transform / Localize.Unit / BB.Message
• BB.Lua.Docs markdown surface (mirrors AshLua.Docs) for bb_mcp / bb_kino consumers

Closes #16.

Test plan

• Review the proposal against ash_lua v0.1.1 to confirm the conventions line up where it matters (Go-style returns, host-supplied context, manifest-driven docs)
• Confirm the error taxonomy matches BB.Command.await/2, BB.Jido.Action.Command, and the sibling bb_python proposal
• Decide on the open questions (VM pooling, argument-type fidelity, async command form, cancellation, hot config reload, expose_commands :all)
• Move to implemented/ once bb_lua ships