[Japabu]

Problem

#1133 measured a real stall in userland's memcpy on the AMD bench (Zen 2). It is closed, because round 12's binding outcome rejected every routed build. The stall it measured, and the cost that sank its fix, need a record so they don't vanish with the PR.

Change

Record-only: adds issues/a-userland-forward-copy-a-short-way-above-its-source-modulo-4-kib-stalls-on-amd.md (kind: finding). It holds:

• the vendor fact, 56305 rev. 3.02 §2.12 p. 39;
• the AMD stall, from 256 B to 64 MiB, with the T14's unexplained window;
• what libc: a forward copy a short way above its source modulo 4 KiB copies down #1133's descending copy fixed;
• the cost every routed build carried: about 7% on never-routed 256 KiB copies, with or without the AVX loop, and not reproduced by moving base's memcpy;
• an exit for a future approach, judged on the same paired-boot rule.

Checks

• cargo test --lib -- sourcegate::tests::no_tracked_file_identifies_a_machine_or_its_network with the file tracked: exit 0.
• No code changes, so no host CI or guest run.
• Every number in the file cites its round on ToyOSOrg/evidence branch pr-1133.

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