feat: genre starters for asobi init --template

[Taure]

What

Adds local genre starters to asobi init --template alongside the existing engine demos:

--template	Result
arena	Real-time movement + server-owned combat (shoot the nearest enemy in range)
chat	A real-time chat room; messages are relayed by the server so the author can't be forged
turn-based	Two players alternate claiming cells on a 3x3 board; out-of-turn / duplicate moves rejected server-side
world	A persistent shared world (game_type = "world", the world-bridge callbacks)
(none) / basic	The existing minimal movement starter
defold / godot / unity / backend	Unchanged - still fetch the full demo repo

Each genre scaffolds a runnable lua/match.lua shaped for that style of game, so a first-timer picks a genre and gets working, genre-appropriate server Lua - offline and instantly.

How

• Templates are embedded files (go:embed) under internal/scaffold/templates/<genre>/, not Go string consts, so the Lua stays real and luac-checkable.
• scaffold.Init(dir, genre) walks the genre's embedded tree; Genres() / IsGenre() back the help and error messages.
• cmdInit routes a genre name to the local scaffold and an engine name to the remote fetch; an unknown name errors listing both sets.
• init next-steps now lead with asobi dev (run locally, no login) before the deploy sequence.

Every genre's callback surface is grounded in the real asobi_lua bridges: match modes define init/join/leave/handle_input/tick/get_state; the world mode uses spawn_position/zone_tick/handle_input(entities)/post_tick. All five match.lua files pass luac -p.

Tests

go build, go vet, gofmt -l, and go test ./... all green. New tests cover the default scaffold, overwrite refusal, unknown-genre error, and a table test asserting every genre scaffolds a lua/match.lua with its bridge's required callbacks plus a README that runs and deploys.