Manifest fields

The keywords a manifest.org declares. Required unless marked optional.

keyword	meaning
`#+TOOLKIT`	slug `= dir name =` id
`#+VERSION`	semver
`#+EXEC`	invocation shape (see below)
`#+TRUST`	first-party (default) / third-party
`#+CAPS`	space-list of Dock capabilities (optional)
`#+BUILD_LANG`	rust / c / zig / js / ts / go
`#+BUILD_SRC`	crate:<n> / path:<dir> / src
`#+CLI_BIN`	command name (for `#+EXEC: command`)
`#+WIT`	WIT world (component / kernel)
`#+ENTRY`	exported entry fn (component / kernel)
`#+AUTHOR_DID`	author identity, for signing (third-party)