A value in an any-typed slot loses its concrete type at save; the document must carry it #712

Epic: #443 — Serialization and the single codec
Related: #711 (declared-type numbers), #709 (conversion rules)

A parameter declared any has no target type to read a serialized value against. Unlike #711, this cannot
be fixed by reading more carefully — the information is destroyed at save, before anything reloads.

The mechanism

json.Marshal(float64(42)) emits 42. Go's encoder writes the shortest representation that round-trips as a
float64, and for an integral value that has no decimal point. So a Starlark float 42.0 landing in a
map[string]any is written to the document as 42, indistinguishable from an integer.

UseNumber on the way back in preserves the literal faithfully. The literal simply no longer says "float".

yaml.v3 marshals float64(42) as 42 as well, so this is not a JSON-specific defect.

It is live

any appears in parameters across nine files, including ones that certainly reach a serialized graph:

plan.Variable(name string, defaultValue any, field string)
plan.Plan(name op.ActionName, args []any, kwargs map[string]any)
template.RenderText(content string, data map[string]any)
function.Call(callable Resource, args []any, kwargs map[string]any)

plan.Variable's defaultValue is the clearest case: a declared default of 1.0 is written as 1 and
reloads as an integer.

Ruling

For a value whose slot is any, the document stores the type. There is no other option: with no
declared type at either end, the document is the only place the information can live.

This does not apply to a value whose parameter has a declared type. There the field says how to read the
number, which is what #711 implements — fs.FileMode wants an integer and the text answers. Only any slots
need the type written down.

What this opens

• The envelope's shape. {"$type": "float", "value": 42} is the obvious form, and it changes every
  document containing an any slot. A narrower alternative — force a decimal point when marshaling a float
  into an any slot, so 42.0 stays 42.0 — is smaller but only covers numbers, and the same question
  arises for any other type whose identity JSON cannot carry.
• Which types need it. Numbers are the case in hand. Strings, bools, lists, and dicts survive JSON
  unambiguously. Anything with a Go type that JSON flattens is a candidate.
• Checksums and graph identity. Changing what a document contains changes the bytes. Whether it changes
  the canonical form is a separate question, and one JSON reload turns every authored integer into a float64, and the codecs disagree #711 is already having to answer.
• Read compatibility. Greenfield, per the repository's governing principle, so old documents need no
  accommodation — but that should be a stated decision rather than an assumption.

Not blocking #711

#711 narrows to parameters with a declared type. any-typed slots keep today's behavior until this is
decided, so the two do not contend.

Acceptance

• A float in an any slot reloads as a float
• An integer in an any slot reloads as an integer
• plan.Variable(default=1.0) survives save and reload with its type intact
• The envelope is documented as part of the document format, not left implicit in the encoder
• JSON and YAML agree

Plan and design documents

Epic: #443 · Feature: #458