fix(agent): extend checked/unknown treatment to input/output token accumulation

input and output token accumulators used saturating_add throughout,
while the PR's own total and cache counters already used checked_add
with Unknown/Poisoned poison semantics (flagged by Wes's agent review).

Introduce TurnIOState (Unseen | Exact(n) | Poisoned) in types.rs:
- fold_round(n): overflow -> Poisoned, absence handled by caller
- merge_session(turn): Poisoned poisons permanently; Exact checked-add
- exact_value(): Some(n) only for Exact

Wire changes:
- accumulatedInputTokens / accumulatedOutputTokens: emitted only when
  Exact; omitted (never null, never u64::MAX) when Poisoned
- used: saturating fallback to 0 for display-only proxy (ACP dead code)

ACP changes:
- UsageUpdatePayload.accumulated_{input,output}_tokens: Option<u64>
  with #[serde(default)] for backward compat; None = publisher-poisoned
- absent field -> delta_reliable: false, null turn fields, null cumulative
- SessionState.last_{input,output}: Option<u64> to match provenance
- TurnUsage.cumulative_{input,output}_tokens: Option<u64>
- pool.rs: pass through directly (TokenCounts fields already Option<u64>)
- Box<UsageUpdatePayload> in GooseSessionUpdateVariant to satisfy
  clippy::large_enum_variant after struct grew with new doc+Option fields

Tests added (426+686 pass):
- TurnIOState unit tests: fold overflow, merge_session overflow, exact_value
- wire: omit input/output when poisoned; emit when exact
- ACP: absent input/output -> unreliable; goose payload unchanged;
  round-trip: poison mid-session -> subsequent turns stay unknown

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

Author: Duncan

crates/buzz-acp/src/acp.rs

@@ -1860,8 +1860,8 @@ impl AcpClient {
                     tracing::debug!(
                         target: "acp::usage",
                         session_id = %notif.session_id,
-                        input = payload.accumulated_input_tokens,
-                        output = payload.accumulated_output_tokens,
+                        input = ?payload.accumulated_input_tokens,
+                        output = ?payload.accumulated_output_tokens,
                         // A subset of `input`, logged so downstream accounting can
                         // price it at the provider's cached rate. Always emitted,
                         // including as 0, so a parser can tell "no cache hits"
@@ -4353,8 +4353,8 @@ mod tests {
         assert_eq!(usage.session_id, "s1");
         assert_eq!(usage.turn_seq, 1);
         assert!(!usage.delta_reliable, "first turn must be unreliable");
-        assert_eq!(usage.cumulative_input_tokens, 1000);
-        assert_eq!(usage.cumulative_output_tokens, 200);
+        assert_eq!(usage.cumulative_input_tokens, Some(1000));
+        assert_eq!(usage.cumulative_output_tokens, Some(200));
         assert_eq!(usage.cumulative_cost_usd, Some(0.01));
 
         // Second take must be None.

crates/buzz-acp/src/pool.rs

@@ -3698,8 +3698,8 @@ pub(crate) fn build_turn_metric_counts(
         None
     };
     let cumulative_counts = Some(TokenCounts {
-        input_tokens: Some(usage.cumulative_input_tokens),
-        output_tokens: Some(usage.cumulative_output_tokens),
+        input_tokens: usage.cumulative_input_tokens,
+        output_tokens: usage.cumulative_output_tokens,
         // Present when every turn in the session reported a genuine provider
         // total. None when the session has never emitted one or any turn lacked
         // one. Never derived from input+output (NIP-AM MUST NOT).
@@ -6168,8 +6168,8 @@ mod tests {
             turn_cost_usd: None,
             turn_cache_read_tokens: None,
             turn_cache_write_tokens: None,
-            cumulative_input_tokens: 100,
-            cumulative_output_tokens: 50,
+            cumulative_input_tokens: Some(100),
+            cumulative_output_tokens: Some(50),
             cumulative_total_tokens: None,
             cumulative_cost_usd: None,
             cumulative_cache_read_tokens: None,
@@ -6207,8 +6207,8 @@ mod tests {
             turn_cost_usd: Some(0.001),
             turn_cache_read_tokens: None,
             turn_cache_write_tokens: None,
-            cumulative_input_tokens: 200,
-            cumulative_output_tokens: 80,
+            cumulative_input_tokens: Some(200),
+            cumulative_output_tokens: Some(80),
             cumulative_total_tokens: None,
             cumulative_cost_usd: Some(0.001),
             cumulative_cache_read_tokens: None,
@@ -6247,8 +6247,8 @@ mod tests {
             turn_cost_usd: None,
             turn_cache_read_tokens: None,
             turn_cache_write_tokens: None,
-            cumulative_input_tokens: 150,
-            cumulative_output_tokens: 70,
+            cumulative_input_tokens: Some(150),
+            cumulative_output_tokens: Some(70),
             cumulative_total_tokens: None,
             cumulative_cost_usd: None,
             cumulative_cache_read_tokens: None,
@@ -6287,8 +6287,8 @@ mod tests {
             turn_cost_usd: None,
             turn_cache_read_tokens: None,
             turn_cache_write_tokens: None,
-            cumulative_input_tokens: 400,
-            cumulative_output_tokens: 100,
+            cumulative_input_tokens: Some(400),
+            cumulative_output_tokens: Some(100),
             cumulative_total_tokens: None,
             cumulative_cost_usd: None,
             cumulative_cache_read_tokens: None,
@@ -6324,8 +6324,8 @@ mod tests {
             turn_cost_usd: None,
             turn_cache_read_tokens: None,
             turn_cache_write_tokens: None,
-            cumulative_input_tokens: 500,
-            cumulative_output_tokens: 120,
+            cumulative_input_tokens: Some(500),
+            cumulative_output_tokens: Some(120),
             cumulative_total_tokens: Some(620), // genuine cumulative total
             cumulative_cost_usd: None,
             cumulative_cache_read_tokens: None,
@@ -6376,8 +6376,8 @@ mod tests {
             turn_cost_usd: None,
             turn_cache_read_tokens: None,
             turn_cache_write_tokens: None,
-            cumulative_input_tokens: 200,
-            cumulative_output_tokens: 60,
+            cumulative_input_tokens: Some(200),
+            cumulative_output_tokens: Some(60),
             cumulative_total_tokens: None, // session has no total
             cumulative_cost_usd: None,
             cumulative_cache_read_tokens: None,