[state] Log pending-buffer evictions instead of silently discarding

[intendednull]

Parent: #108

Problem

crates/state/src/sync.rs:177-186:

pub fn buffer_for_prev(&mut self, prev_hash: EventHash, event: Event) {
    self.waiting_on_prev
        .entry(prev_hash)
        .or_default()
        .push(event);
    self.cached_count += 1;
    if let Some(limit) = self.max_pending {
        self.evict_to(limit);
    }
}

evict_to returns the number of evicted events — buffer_for_prev calls it and discards the return value. Nothing logs when pending events are dropped under capacity pressure. A gap in the DAG chain can be silently lost.

Fix

Capture the return value and log it:

if let Some(limit) = self.max_pending {
    let evicted = self.evict_to(limit);
    if evicted > 0 {
        tracing::warn!(
            evicted,
            buffer_size = self.cached_count,
            "pending buffer at capacity; dropped oldest events"
        );
    }
}

Optional follow-up: consider returning the evicted events from buffer_for_prev so the sync layer can re-request them later. That's larger and can be a separate issue.

Test

Add a test that inserts more events than max_pending, then asserts:

1. The tracing span emits a warning. (Use tracing-test or a custom subscriber.)
2. cached_count matches the cap.
3. evicted > 0 is returned if you change the return type.

Note

The BTreeMap::keys().next() eviction order is already deterministic (this was flagged by the review agent but is a false positive — BTreeMap iterates in sorted key order). No change needed there.

[intendednull]

Fixed. crates/state/src/sync.rs:183-192 now captures evict_to's return value and emits tracing::warn!(evicted, buffer_size, "pending buffer at capacity; dropped oldest events") whenever events are dropped. Regression test lives in crates/state/src/tests.rs:2856 (pending_buffer_eviction_reduces_count_to_cap) under the "Issue #123: PendingBuffer eviction logging" section.

[intendednull]

Already implemented in commit ab78113 (merged via #146): PendingBuffer evictions now logged at warn! instead of silently discarding. Closing as resolved.