feat: v2.5 — arbitrary capacity, async crate, pipeline wait strategy, derive fix, metrics, loom

[userFRM]

Summary

Eliminates 7 cons from the photon-ring audit. 26 files changed, 2,246 additions across the core crate + 2 new companion crates.

1. Arbitrary ring capacity (Lemire fastmod)

• Before: capacity must be power-of-two (seq & mask)
• After: any capacity >= 2 (reciprocal-multiply fastmod, ~1.5ns)
• Power-of-two users get zero regression (is_pow2 branch, perfectly predicted)
• 15 new tests including exhaustive fastmod verification for capacities 2-50

2. Pipeline WaitStrategy

• then_with(f, WaitStrategy::YieldSpin) — configurable wait per stage
• then_a_with(), then_b_with() for fan-out branches
• Existing then()/then_a()/then_b() unchanged (delegate with default)
• 5 new tests

3. Derive macro #[photon(as_enum)]

• Before: unknown types silently fell to FieldKind::Enum → confusing transmute errors
• After: compile error for unrecognized types + explicit #[photon(as_enum)] attribute
• Breaking change for Message derive users with enum fields (add the attribute)

4. photon-ring-async (new crate)

• Runtime-agnostic — no tokio, no async-std, just core::future
• AsyncSubscriber, AsyncSubscriberGroup with yield-based polling
• Named RecvFuture/GroupRecvFuture for select!/join!
• Configurable spin_budget, 8 tests, pollster test executor

5. photon-ring-metrics (new crate)

• SubscriberMetrics with snapshot() and delta() tracking
• PublisherMetrics::snapshot()
• Display impl for logging, 7 tests, zero runtime dependencies

6. Loom MPMC testing

• Standalone loom model of the MPMC cursor advancement protocol
• 4 scenarios: 2-producer basic, contention, consumer read, catch-up
• Run manually: cargo test --test loom_mpmc --release (too slow for CI)

7. Dead code cleanup

• Removed #[allow(dead_code)] from ring.rs → #[cfg(test)]

Test plan

• cargo test --workspace — 153 tests pass
• cargo clippy --workspace --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• CI pipeline passes

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