fix: clear CI mypy + rust test failures introduced in eaf5980

compression_units.py:
- Replace dict-unpacking pattern with dataclasses.replace() so mypy can
  type-check fields. The `**base` form forced mypy to infer
  `dict[str, object]`, which doesn't satisfy the per-field types of
  UnitCompressionResult (46 arg-type errors).
- Use `isinstance(candidates, Iterable)` for the transform-iteration
  guard. The previous `iter()` call had a `# type: ignore[arg-type]`
  that was misclassified — mypy actually emits `call-overload` here.

live_zone_thresholds.rs:
- Update the JsonArray threshold assertion from 1024 to 512 to match
  the new constant. eaf5980 lowered THRESHOLD_JSON_ARRAY from 1024 → 512
  in live_zone.rs but missed this integration test.

crates/headroom-core/tests/live_zone_thresholds.rs

@@ -51,14 +51,14 @@ fn first_tool_result_action(out: &LiveZoneOutcome) -> BlockAction {
 
 #[test]
 fn below_threshold_no_compression_attempted() {
-    // 200 bytes of homogeneous JSON dicts — well below the 1 KiB
+    // 200 bytes of homogeneous JSON dicts — well below the 512 B
     // JsonArray threshold. The dispatcher must record
     // `BelowByteThreshold` and emit `NoChange`; no compressor runs.
     let small_array: Vec<Value> = (0..3).map(|i| json!({"id": i, "v": "x"})).collect();
     let payload = serde_json::to_string(&small_array).unwrap();
     assert!(
-        payload.len() < 1024,
-        "fixture must stay below the 1 KiB JsonArray threshold; got {}",
+        payload.len() < 512,
+        "fixture must stay below the 512 B JsonArray threshold; got {}",
         payload.len()
     );
 
@@ -80,7 +80,7 @@ fn below_threshold_no_compression_attempted() {
         } => {
             assert_eq!(content_type, "json_array");
             assert_eq!(byte_count, payload.len());
-            assert_eq!(threshold_bytes, 1024);
+            assert_eq!(threshold_bytes, 512);
         }
         other => panic!("expected BelowByteThreshold for sub-threshold JSON, got {other:?}"),
     }

headroom/transforms/compression_units.py

@@ -9,7 +9,7 @@ replacements back into their native request shape.
 from __future__ import annotations
 
 from collections.abc import Iterable
-from dataclasses import dataclass, field
+from dataclasses import dataclass, field, replace
 from typing import Protocol
 
 from .content_router import CompressionStrategy, ContentRouter, RouterCompressionResult
@@ -63,11 +63,9 @@ def find_content_router(transforms: object) -> ContentRouter | None:
     """Return the first ContentRouter in a pipeline or iterable."""
 
     candidates = getattr(transforms, "transforms", transforms)
-    try:
-        iterator = iter(candidates)  # type: ignore[arg-type]
-    except TypeError:
+    if not isinstance(candidates, Iterable):
         return None
-    for transform in iterator:
+    for transform in candidates:
         if isinstance(transform, ContentRouter):
             return transform
     return None
@@ -86,32 +84,32 @@ def compress_unit_with_router(
     """
 
     tokens_before = tokenizer.count_text(unit.text)
-    base = {
-        "original": unit.text,
-        "compressed": unit.text,
-        "modified": False,
-        "tokens_before": tokens_before,
-        "tokens_after": tokens_before,
-        "tokens_saved": 0,
-        "transforms_applied": [],
-        "strategy": CompressionStrategy.PASSTHROUGH.value,
-        "router_result": None,
-    }
+    base = UnitCompressionResult(
+        original=unit.text,
+        compressed=unit.text,
+        modified=False,
+        tokens_before=tokens_before,
+        tokens_after=tokens_before,
+        tokens_saved=0,
+        transforms_applied=[],
+        strategy=CompressionStrategy.PASSTHROUGH.value,
+        router_result=None,
+    )
 
     if not unit.mutable:
-        return UnitCompressionResult(**base, reason="immutable")
+        return replace(base, reason="immutable")
     if unit.role == "user":
-        return UnitCompressionResult(**base, reason="protected_user_message")
+        return replace(base, reason="protected_user_message")
     if unit.role in {"system", "developer"}:
-        return UnitCompressionResult(**base, reason="protected_system_message")
+        return replace(base, reason="protected_system_message")
     if unit.role == "assistant" and unit.metadata.get("compress_assistant") != "true":
-        return UnitCompressionResult(**base, reason="protected_assistant_message")
+        return replace(base, reason="protected_assistant_message")
     if unit.cache_zone != "live":
-        return UnitCompressionResult(**base, reason=f"cache_zone_{unit.cache_zone}")
+        return replace(base, reason=f"cache_zone_{unit.cache_zone}")
     if len(unit.text) < unit.min_bytes:
-        return UnitCompressionResult(**base, reason="below_unit_floor")
+        return replace(base, reason="below_unit_floor")
     if "Retrieve more: hash=" in unit.text or "Retrieve original: hash=" in unit.text:
-        return UnitCompressionResult(**base, reason="already_compressed")
+        return replace(base, reason="already_compressed")
 
     router_result = router.compress(
         unit.text,
@@ -122,21 +120,21 @@ def compress_unit_with_router(
     replacement = router_result.compressed
     strategy = router_result.strategy_used.value
     if replacement == unit.text:
-        return UnitCompressionResult(
-            **{**base, "strategy": strategy, "router_result": router_result},
+        return replace(
+            base,
+            strategy=strategy,
+            router_result=router_result,
             reason="router_no_change",
         )
 
     tokens_after = tokenizer.count_text(replacement)
     if tokens_after >= tokens_before:
-        return UnitCompressionResult(
-            **{
-                **base,
-                "compressed": replacement,
-                "tokens_after": tokens_after,
-                "strategy": strategy,
-                "router_result": router_result,
-            },
+        return replace(
+            base,
+            compressed=replacement,
+            tokens_after=tokens_after,
+            strategy=strategy,
+            router_result=router_result,
             reason="rejected_not_smaller",
         )