fix(swiftpm): actually read the artifacts version pin back

[chrfalch]

Summary:

spm add --version <ver> pinned a value that nothing ever read back.

The marker write has been there all along, and so has the reader — readArtifactsVersionOverride() in spm/generate-spm-xcodeproj.js, exported and unit-tested. It just had zero production callers. Every reference to it was a test.

determineVersion — the only resolver — went straight from the flag to package.json:

let version = args.version;      // --version
if (version == null) {
  version = pkgJson.version;     // react-native/package.json; marker never consulted
}

That value picks which artifact slots the project gets wired to. So:

1. spm add --version 0.88.0-nightly-… → wires the nightly's slots, pins the label ✅
2. spm update (no flag) → silently resolves package.json's version instead, re-pointing the project at different slots, while the marker still claims the nightly ❌

--version was effectively single-use. In this monorepo package.json is 1000.0.0, which has no published artifacts, so a flagless run after a pinned add fails outright — which is why the standing advice has been to pass --version on every invocation. That advice was working around this bug.

Fix: insert the pin between the two existing sources.

--version  →  pinned override  →  react-native/package.json

15 lines of logic. spm download and the scaffold path pick it up for free, since both consume the same resolved value. Because this is persistent state, one line is logged when the pin is the source, so a stale pin is diagnosable instead of silent; there is still no way to clear it short of deinit.

Three comments were actively wrong and are corrected here: the reader's doc block, the marker-field comment, and findInjectedXcodeproj's comment all asserted that the build-time sync calls this via readArtifactsVersionOverride. It does not and never did — the sync action returns before artifacts are resolved at all. Those comments are what made the gap invisible; they misled me while investigating.

Changelog:

[Internal] [Fixed] - SwiftPM: spm --version now sticks, so a later run without the flag keeps using the pinned artifact version

Test Plan:

yarn jest packages/react-native/scripts → 31 suites, 684 tests, all green.

New tests, written red first — the load-bearing one failed with Expected: "0.80.0" / Received: "1000.0.0", i.e. exactly the reported bug:

• --version given → wins, even with a different value pinned
• no flag, pin present → the pinned version is used
• no flag, no pin → package.json's version (unchanged behaviour)
• no flag, corrupt or absent marker → package.json's version, no throw
• the log line fires only when the pin is the source

The three fallback cases passed before the fix too, which is the point — they pin today's behaviour so this change can't regress it.

Note on landing order

Touches setup-apple-spm.js and spm/generate-spm-xcodeproj.js, which #57744, #57756 and #57757 also touch. All are cut independently from main; whichever lands first, I'll rebase the rest. #57756 is the closest relative — it does the same wiring for --config-command, which had the identical write-only-pin shape.