Let a remote loader read a flag the local layer carries

[andyyhope]

What

Adds EnvironmentConfiguration.fetchRemote(_:), so the remote layer can be fetched from a URL that itself lives in a flag the local config sets.

Why

Some apps store the remote config's endpoint in their bundled local config: the app only knows where to fetch from after the local layer is applied. load(_:) already applies the local layer in full before running the remote loader, so the timing was never the problem — the pole was. It doesn't exist when the configuration is built (the pole is constructed from config.sources), so a loader passed to init has nothing to resolve a flag against.

fetchRemote(_:) closes that construction-order knot: set the loader after the pole exists, capturing it.

let config = EnvironmentConfiguration(AppFlags.self, local: { env in bundledJSON(env) })
let pole = FlagPole(AppFlags.self, sources: [byHand] + config.sources)

config.fetchRemote { [pole] env in
    let url = pole.flags.checkout.remoteURL   // by-hand ▸ (remote cleared) ▸ local ▸ default
    return try await api.fetch(url)
}

Because the loader resolves through the whole pole, a by-hand override of the URL beats the bundled value — so a staging build can be pointed at a different config server from the companion without a rebuild.

API

• New: EnvironmentConfiguration.fetchRemote(_:).
• The remote: init parameter is now optional (defaults to a nil-returning loader). A nil loader leaves the remote layer .absent. Non-breaking — existing remote:-at-init call sites still compile.

Notes

• The loader is snapshotted under the existing lock in load(_:), alongside epoch; remote moved from let to var.
• Overlapping switches: a slow in-flight fetch may read the newer environment's URL, but the epoch guard drops its result as .superseded — a wasted fetch, never a wrong apply. Same guarantee as before.
• Demo left unchanged: its remote config is genuinely fixed-per-environment, so fetchRemote there would be an artificial example.

Tests

Three added to EnvironmentConfigurationTests (TDD, red first):

• loader runs after the local layer is applied,
• loader resolves through a by-hand override sitting above local,
• no loader → remote layer .absent.

Full suite: 774 pass, 0 failures. DocC builds clean under --warnings-as-errors (new subsection in SourcesAndPrecedence.md).

🤖 Generated with Claude Code

https://claude.ai/code/session_01NnwCqKNm8rZ63Au6bpYdaD