Avoid strong reference reader count overflow

[fallintoplace]

Summary

• Prevent atomic strong-reference reader counts from wrapping at capacity.
• Replace masking/assert-only reader count encoding with checked preconditions.
• Add a checked reader increment path that returns nil when the reader counter is full, causing _startLoading to reload and retry instead of publishing a wrapped count.
• Add a focused regression test using a tiny injected reader mask through an underscored testing SPI.

Why

The strong-reference storage reserves a small reader-count field inside DoubleWord and previously incremented it with wrapping arithmetic. If enough readers entered concurrently, the count could wrap and make the storage appear to have fewer active readers than it actually did.

Testing

• swift build passes locally.
• swift test --filter StrongReferenceRace.testReaderCountDoesNotWrapAtCapacity could not run locally because this Swift toolchain cannot import XCTest.