Remove Identifiers; Fake.xid() becomes a seeded opaqueID()

[verygoodsoftwarenotvirus]

Closes #5.

Why

primitives-conformance turned up that the four ports disagree on what identifiers means. Swift was the port that agreed — XID.swift is a real hand-written xid (12 raw bytes, machine ID, timestamp encoding), and Tests/IdentifiersTests/XIDInteropTests.swift existed specifically to check it against Go's. It passed.

Agreeing with Go by accident is still not a reason to carry a reimplementation of Go's ID format on a handset. A client has no business generating a server's identifiers — the server issues them, the client receives opaque strings, and validating an ID the server just sent you proves nothing. So identifiers stays a primitives-go package and all three non-Go ports drop it (ts: primitives-ts#76, kt: primitives-kt#9).

That an interop test suite already existed for this is the tell: the concern was live, and the answer was to keep two implementations in lockstep forever rather than to ask whether the second one should exist.

Fake.xid() → Fake.opaqueID()

Fake was the only consumer, two call sites behind a public xid(). A fake ID that looks like a server ID is genuinely useful in a client's tests, so the capability stays and the semantics go:

• 20 lowercase base32-hex characters — the right shape for test data.
• No timestamp, no machine identity, no parsing. It is a plausible stand-in for an ID the server would have sent, not an xid.
• Drawn entirely from the seeded source. The old xid() was documented as not seed-reproducible (FakeGenerator's type-level docs called it out by name); opaqueID() is. That doc block is updated, and FakeTests' reproducibility case now calls opaqueID() so the claim is exercised rather than asserted — it wasn't in that test before.

This is option 1 from #5. Option 2 (delete it outright) would push every test that wanted an ID-shaped value into hand-rolling one.

Also in here

Package.swift still declared name: "platform-swift" while README.md — updated in #3 — already told consumers package: "primitives-swift". One line, a leftover from the rename, fixed here because leaving a manifest and its README disagreeing is worse than the small scope creep. Say the word if you'd rather it went separately.

Verification

swift build clean · swift test 1133 tests in 246 suites passing (1152 before, minus the 19 IdentifiersTests).

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