Streaming partials for @Extractable

[asaptf]

Closes the last open feature on the roadmap. A caller can now show fields filling in as the model produces them instead of waiting on a spinner.

let stream = Extract.stream(from: pdfURL, as: Receipt.self, using: session)
for try await update in stream {
    switch update {
    case .partial(let p):   // Receipt.Partial — every field optional
    case .final(let r):     // ExtractionResult<Receipt> — identical to `detailed`
    }
}

The rule this feature is built around

A partial never shows a value that a later snapshot contradicts within the same token. While the model is writing 473.00, the caller does not see 47 — a wrong total on screen is worse than an empty field. A scalar surfaces only once its JSON token is provably complete: closing quote for strings, a delimiter after a number, the full keyword for true/false/null.

This trades latency for truthfulness deliberately, and DECISIONS.md says so explicitly so it does not get "optimised" later.

What the assembler actually emits mid-stream, now locked down by exact assertions:

Buffer	Snapshot
{"total":473	{}
{"total":473.00	{}
{"total":473.00}	{"total":473.00}
{"name":"Ada","total":47	{"name":"Ada"}
{"lineItems":[{"sku":"A","qty":1	{"lineItems":[]}
{"lineItems":[{"sku":"A","qty":1}	{"lineItems":[{"sku":"A","qty":1}]}

Arrays grow element by element as each element completes.

Honest limits, documented rather than papered over

• Chunked documents emit no partials, only .final. Per-chunk snapshots are incoherent before the deterministic merge, and we have measurements showing what that merge is worth.
• Signals, provenance, grounding and tables attach to .final only. A partial is a UI preview, not an evidenced result.
• Repair retries are not streamed. Partials come from the first attempt; the stream still ends with .final, or throws the same errors as detailed.

Compatibility

Additive throughout. Extractable gains associatedtype Partial: Decodable & Sendable = Self, so hand-written conformances keep compiling; the macro generates a real Partial for @Extractable types. The ExtractionGenerating seam gains a streaming method with a default implementation that calls the existing generate once, so every existing test conformance and MockLanguageModel are untouched.

Verification

• swift build, swift test — 190 tests; the pre-existing 177 pass unchanged
• swift format lint --strict — silent
• Macro expansion tests cover nested types and arrays

A first pass of the regression tests guarded their assertions behind if let snap { … }, which would have passed vacuously had the parser returned nil — including if it drifted toward emitting nothing at all. Those are the tests protecting the rule above, so they were rewritten to assert exact snapshots in both directions, and a missing case was added: a completed field alongside an incomplete one, which is the realistic mid-stream shape neither original test covered.