Design: building a module, and why the recipe cannot always be a Dockerfile

12-a-module-repository says what a module may build and where it goes. Nothing
said how a build is MODELLED, and the model is the problem: a recipe is implicit,
singular and always a Dockerfile; a toolchain is not modelled at all, arriving as
two build arguments the module hand-writes; a language is not a concept; and an
archive is declared in the manifest and refused by the builder.

The cost is measurable rather than theoretical. Adding a module with its own code
means repeating an incantation - two ARG bases, a specific working directory so
the SDK resolves upward, the compiler invoked by absolute path because the usual
symlink is resolved away when the base is assembled, a second stage, an env var
naming the entrypoints. Most of the catalogue is unconverted, and two conversions
done in one session were each wrong twice with a working example open.

So: recipe becomes explicit with three kinds, and toolchain becomes derived from
a declared language rather than written by every author. A Dockerfile stays, and
stops being compulsory - it is right for software needing a particular base and
wrong for "compile my module's code", which is the same operation every time.

The cost is stated before it is chosen: every language is permanent, and the
contracts are already expressed twice - Go structs and TypeScript types kept in
step by hand. A second language makes that drift. So language-neutral contracts
come first, or the drift gets worse while hiding.

Claude-Session: https://claude.ai/code/session_01D6qtiYU3P9jk3pnAXyAFyx

03-DESIGN/01-to-be/README.md

@@ -27,6 +27,7 @@ document is written and this one's status becomes `implemented`.
 | [`15-the-agent-session.md`](15-the-agent-session.md) | One mechanism started twice — a node's session and the mesh's | [ADR 0004](../../02-DECISIONS/0004-a-node-and-how-it-joins.md), [ADR 0026](../../02-DECISIONS/0026-the-mesh-has-a-session-of-its-own.md) |
 | [`16-module-coverage.md`](16-module-coverage.md) | What a module must be able to say, measured against 127 that exist | [ADR 0009](../../02-DECISIONS/0009-modules-and-the-graph.md), [ADR 0005](../../02-DECISIONS/0005-the-node-host.md) |
 | [`17-raising-a-mesh.md`](17-raising-a-mesh.md) | How a mesh comes into existence, and how a machine joins one that exists | [ADR 0067](../../02-DECISIONS/0067-genesis-is-a-pivot.md), [ADR 0006](../../02-DECISIONS/0006-the-substrate-and-the-control-plane.md), [ADR 0005](../../02-DECISIONS/0005-the-node-host.md) |
+| [`18-building-a-module.md`](18-building-a-module.md) | How a build is modelled, and why a recipe that is always a Dockerfile does not fit what a module is | [ADR 0040](../../02-DECISIONS/0040-what-a-module-is.md), [ADR 0039](../../02-DECISIONS/0039-what-the-sdk-holds-and-refuses.md), [ADR 0009](../../02-DECISIONS/0009-modules-and-the-graph.md) |
 
 ## Not yet written