feat(cog-pose-estimation): x86_64 release v0.0.1 — parallel to arm (#643)

Adds the x86_64-unknown-linux-gnu binary uploaded to
gs://cognitum-apps/cogs/x86_64/, signed with the same Ed25519
COGNITUM_OWNER_SIGNING_KEY as the arm release. Together with the
already-shipped arm artifact, the cog now ships natively for both
target architectures the Cognitum fleet supports.

x86_64 release:
  sha256:    a434739a24415b34e1aff50e5e1c3c32e568db96af473bbb3e5ecc9b95fe71fa
  signature: pNNuxhgM18PztN8BSZdfw5oAShG2pV3na5T/q2QdlJWX/5FJgo4QTiUCbcTAxI2Uiva8VURSOlRzMU3xoQPqCQ==
  size:      4,548,856 bytes
  cold-start: 5.4 ms / invocation on ruvultra (RTX 5080, NVMe)

Reorganizes manifests under cog/artifacts/manifests/{arm,x86_64}/
so each arch carries its own manifest with the matching binary_sha256
and signature — same layout the release pipeline will use for the
future hailo8 / hailo10 variants.

Updates docs/benchmarks/pose-estimation-cog.md with the cross-arch
cold-start table:

  Windows (x86_64)   76.2 ms
  ruvultra (x86_64)   5.4 ms   <- this release
  Pi 5 (aarch64)     8.4 ms

Verified via anonymous GCS download + SHA round-trip — identical to
local build.

Hailo HEF remains the only pending arch, still blocked on Hailo SDK
provisioning to a self-hosted runner.

v2/crates/cog-pose-estimation/cog/artifacts/manifests/x86_64/manifest.json

@@ -0,0 +1,28 @@
+{
+  "id": "pose-estimation",
+  "version": "0.0.1",
+  "binary_url": "https://storage.googleapis.com/cognitum-apps/cogs/x86_64/cog-pose-estimation-x86_64",
+  "binary_bytes": 4548856,
+  "binary_sha256": "a434739a24415b34e1aff50e5e1c3c32e568db96af473bbb3e5ecc9b95fe71fa",
+  "binary_signature": "pNNuxhgM18PztN8BSZdfw5oAShG2pV3na5T/q2QdlJWX/5FJgo4QTiUCbcTAxI2Uiva8VURSOlRzMU3xoQPqCQ==",
+  "weights_url": "https://storage.googleapis.com/cognitum-apps/cogs/arm/cog-pose-estimation-pose_v1.safetensors",
+  "weights_bytes": 507032,
+  "weights_sha256": "eb249b9a6b2e10130437a10976ed0230b0d085f86a0553d7226e1ae6eae4b9e5",
+  "arch": "x86_64",
+  "target_triple": "x86_64-unknown-linux-gnu",
+  "installed_at": 0,
+  "status": "installed",
+  "signed_by": "COGNITUM_OWNER_SIGNING_KEY",
+  "sig_algo": "Ed25519",
+  "build_metadata": {
+    "rust": "1.89.0",
+    "candle": "0.9 cpu",
+    "cog_pose_version": "0.3.0",
+    "host": "ruvultra (RTX 5080)",
+    "training_pck20": 3.0,
+    "training_pck50": 18.5,
+    "training_mpjpe_normalized": 0.0931,
+    "cold_start_ms_avg": 5.4,
+    "bench_invocations": 30
+  }
+}