llm: patch 03 - pin the kpool tail block through the unifier

Patch 02 (MLA-multiple max page) cleared the KV page-size unifier: the
dflash boot now reaches cudagraph capture. New blocker: the kpool decode
kernel asserts tail_kv_cache.shape[2] == pool_size; KpoolTailSpec pins
block_size == index_kpool, so the unifier's divisible branch must never
grow it. Patch 03 = 02 + a KpoolTailSpec branch that pads page_size_padded
instead. Apply-verified: reproduces patched sha 5093e4b7... Runbook
updated with the full KV grouping journey.

scripts/patches/glm53-dflash-03-kv-unify-tailpin.patch

@@ -0,0 +1,37 @@
+--- a/vllm/v1/core/kv_cache_utils.py
++++ b/vllm/v1/core/kv_cache_utils.py
+@@ -1395,10 +1395,34 @@
+         return kv_cache_spec
+ 
+     max_page_size = max(page_sizes)
++    # GLM-5.3-Flash + a draft model: the sparse indexer k_cache page is
++    # tokens_per_state x the MLA page (same per-token bytes), and MLA layers
++    # cannot be PADDED to an arbitrary page size (sparse MLA indexes the cache
++    # in whole token rows) -- but they CAN grow their block size when the max
++    # page is an exact multiple of the MLA page. Bump the max page to a
++    # multiple of every MLA-family page so all of them take the divisible
++    # block-growth branch below instead of the padding refusal, while the
++    # draft and mamba layers simply pad to the (slightly larger) page.
++    mla_pages = {
++        spec.page_size_bytes
++        for spec in kv_cache_spec.values()
++        if isinstance(spec, MLAAttentionSpec)
++    }
++    if mla_pages:
++        mla_common = math.lcm(*mla_pages)
++        max_page_size = -(-max_page_size // mla_common) * mla_common
+     new_kv_cache_spec = {}
+     for layer_name, layer_spec in kv_cache_spec.items():
+         if layer_spec.page_size_bytes == max_page_size:
+             new_kv_cache_spec[layer_name] = layer_spec
++        elif isinstance(layer_spec, KpoolTailSpec):
++            # The kpool tail kernel pins the tail block to the kpool size
++            # (tail_kv_cache.view(...) must be [blocks, 2, index_kpool,
++            # head_dim]); growing block_size breaks that contract. Pad the
++            # physical page instead -- the tail is its own group and the pad
++            # wastes bytes only.
++            new_spec = replace(layer_spec, page_size_padded=max_page_size)
++            new_kv_cache_spec[layer_name] = new_spec
+         elif isinstance(layer_spec, MambaSpec):
+             # MambaSpec's page size is determined by its state shapes and does
+             # not scale with block_size, so pad the page instead. This is the