Keep CP HiCache owner lanes padded for valid-tail victims

Load-back owner-lane eviction can need to evict a device-resident node whose radix value is a valid tail shorter than the physical tail page. The eviction planner now pads that value only to the page boundary before deriving owner counts, matching the existing write/load capacity contract without exposing padding to radix or scheduler lengths.

Constraint: CP HiCache capacity remains page-owner based while radix node values remain valid-token based.
Constraint: Avoid collectives; owner-lane capacity must be deterministic from local metadata and logical page ids.
Rejected: Require device-resident victim values to be page-aligned | valid-tail cache nodes are now an intentional supported state.
Rejected: Pad to cp_size pages | this would waste KV and violate the page-boundary-only contract.
Confidence: medium
Scope-risk: narrow
Directive: If split-inside-tail support is added later, preserve page ownership/refcount semantics before sharing one padded physical page across radix nodes.
Tested: local py_compile for hiradix_cache.py and touched CP HiCache tests.
Tested: remote g0034 new C8 exact tests: 3 passed, 3 warnings.
Tested: remote g0034 CP HiCache impacted suites: 146 passed, 5 warnings.
Tested: remote g0034 CP shared KV C1-C5 suite: 122 passed, 5 warnings.
Not-tested: full local pytest, blocked by missing runtime dependencies such as starlette.
Not-tested: CUDA E2E runtime for this commit.
Co-authored-by: OmX <omx@oh-my-codex.dev>

python/sglang/srt/mem_cache/hiradix_cache.py

@@ -1033,14 +1033,16 @@ class HiRadixCache(RadixCache):
         value = getattr(node, "value", None)
         if value is None or int(value.numel()) == 0:
             return tuple(0 for _ in range(cp_size))
-        if int(value.numel()) % self.page_size != 0:
-            raise RuntimeError(
-                "CP HiCache load-back eviction requires page-aligned device values: "
-                f"node_id={getattr(node, 'id', '?')} value_len={value.numel()} "
-                f"page_size={self.page_size}"
-            )
+        padded_value = pad_token_locs_to_page_boundary(
+            value,
+            self.page_size,
+            name=(
+                "CP HiCache load-back eviction device values "
+                f"node_id={getattr(node, 'id', '?')}"
+            ),
+        )
 
-        first_locs = value[:: self.page_size]
+        first_locs = padded_value[:: self.page_size]
         logical_pages = torch.div(first_locs, self.page_size, rounding_mode="floor")
         owners = torch.remainder(logical_pages - 1, cp_size)
         return tuple(int((owners == owner).sum().item()) for owner in range(cp_size))