Docs site 3/6: ABI, async protocol, and hostgen DSL reference

[zklapow]

Part 3 of the docs stack (base: docs-user-guide).

Adds the reference section — the specs that previously existed only as code or example-README fragments:

• Base ABI specification: all guest exports with signatures, return codes, buffer ownership, output protocol, and call sequences, sourced from python-host-core/src/export.rs + PythonInstance.java. This is the documentation half of Base Java↔WASM ABI has no spec and no version handshake #43 (the version-handshake code change remains open there).
• Extension ABI JSON & lowering: the schema, field semantics, WASM signature lowering rules, and the result-buffer protocol.
• Async wire protocol v1: promoted from the custom-python-build README into a canonical page (the example README now links here).
• hostgen DSL reference: the full ExtensionSpec/FunctionSpec/Build surface plus CLI flags.

The rust-host example README's lowering table is replaced with a link to the canonical page.