feat: atomic-slots — formally sound seqlock via AtomicU64 stripes

[userFRM]

Summary

Adds the atomic-slots feature flag that eliminates the seqlock's formal undefined behavior under the Rust abstract machine. Replaces write_volatile/read_volatile with per-u64 AtomicU64::store/load(Relaxed), which compiles to identical MOV instructions on x86-64.

This design was discovered via 3-agent constraint-anchored analysis (prohibition engine + impossibility proofs). All 3 independent agents converged on the same architecture. The research documents are in docs/research-*.md.

How it works

# Opt in to formal soundness
photon-ring = { version = "2.3.0", features = ["atomic-slots"] }

• Default: volatile-based seqlock (fastest, practical UB — same as Linux kernel)
• atomic-slots: AtomicU64 stripes (formally sound, Miri-passable, zero cost on x86-64)

Benchmarks (x86-64, Criterion, 100 samples)

Benchmark	Default	atomic-slots	Delta
Publish only	1.94 ns	1.98 ns	+2% (noise)
Fanout 1 sub	3.95 ns	5.70 ns	+44% (stripe overhead)
Fanout 2 subs	6.21 ns	6.14 ns	-1% (noise)
Fanout 10 subs	15.68 ns	15.96 ns	+2% (noise)
SubscriberGroup	2.89 ns	2.93 ns	+1% (noise)
Cross-thread	96.2 ns	95.2 ns	-1% (noise)
MPMC 1p 1s	12.23 ns	11.99 ns	-2% (noise)

Single-subscriber roundtrip has ~1.7ns overhead from stripe marshaling. Multi-subscriber fanout and cross-thread latency are within noise. On ARM64, one extra DMB ISHLD barrier in the reader (~5-10ns).

Changes

• src/slot.rs: Feature-gated write/write_with/try_read with AtomicU64 stripe protocol
• tests/atomic_slots.rs: 8 new tests (partial stripes, odd sizes, 1M stress, MPMC, bounded)
• Cargo.toml: atomic-slots = [] feature, updated description + keywords
• README.md: Updated tagline, added feature docs, soundness note
• CHANGELOG.md: Feature entry + research documents note
• .github/workflows/ci.yml: New atomic-slots CI job
• 8 docs updated: ROADMAP, benchmark methodology, all 3 research docs, discussion doc, audit doc

Test plan

• cargo test --workspace — all existing tests pass (default mode)
• cargo test --workspace --features atomic-slots — all tests pass
• cargo test --features atomic-slots --test atomic_slots — 8 new tests pass
• cargo clippy --all-features -D warnings — clean
• cargo bench --bench throughput vs --features atomic-slots — benchmarked
• CI pipeline passes
• Multi-model audit on changes (pending)

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