nvx-rs slimming 5.1.2: NVXC v1 and FEATURES are gone #1129

Parent: #1089, in #1076. Needs: #1128. Unblocks: #1131, #1132. No PR of this group merges before the API-ready commit.

Leaving code only. Every PR in this group changes only code that this workstream removes from NVX: NVXC v1's frames, FEATURES and GuestFeatures, with the fake OpenVMM's v1 handler, with the edits that remove references to it. Nothing that stays changes, and no later PR uses what these PRs change, so the size limits for kept code don't apply. Each PR still records its core lines and counted files, and its description opens with this note.

Goal. NVXC v1's frames, FEATURES and GuestFeatures are gone, with the fake OpenVMM's v1 handler and feature state; v2 is NVXC's only version.

PRs, in order. Each PR has one goal, leaves dev green, and names in its description each old test of its area as kept, rewritten or retired, with each change that check_test_changes.py reports. Its tests follow the repository's test layout: a module's unit tests, and the code that only they use, sit in its tests.rs or tests/ and move with it. The estimates count core lines (code, not comments, docs or tests) and counted files; each PR records its measured values. Each PR does what its Scope says and no more: a case, input or layout that its Scope doesn't name, and whatever "Not in this PR" lists, is out of scope. Address every review comment within the scope, and ask before acting on one that asks for more.

1. SL4.4d: NVXC v1 and FEATURES are gone.
   • Objective: After every NVX control path uses v2, v1 and feature advertisement are dead compatibility code. This leaving-code-only PR removes that code and its references so v2 is the sole NVXC protocol.
   • Contents: Leaving code. Delete v1's frames, FEATURES and GuestFeatures, with the fake VMM's v1 handler and feature state. The guest agents no longer serve v1 or FEATURES.
   • Scope:
     • Deletion of NVXC v1 frame constants, codec paths and v1-only tests.
     • Deletion of FEATURES handling and the GuestFeatures type, constants, state and callers.
     • Deletion of the guest agents' v1 serving paths and the fake VMM's v1 handler and feature state; its production and tests.rs changes are flagged and explained.
     • Reference and test updates required for the remaining v2-only code to compile and run.
   • Not in this PR:
     • Rewriting, renaming or cleaning up v2 code that remains: this PR changes only code that leaves and references to it.
     • Changing v2 framing, streaming, capture or restore behavior: SL4.4a through SL4.4c establish those behaviors.
     • Adding compatibility negotiation or a v1 fallback: v2 is the only supported internal version.
     • Incrementing contract-version constants or their generated copies: this workstream does not deploy mixed versions.
     • Deleting unrelated guest, host or fake-VMM code.
   • Approach: Delete the now-unreferenced v1 and FEATURES branches in place, then remove imports, state and tests that mention them. Keep v2 code byte-for-byte where reference removal does not force an edit; do not combine the deletion with refactoring or protocol changes.
   • Done when:
     • No NVXC v1, FEATURES or GuestFeatures code or caller remains.
     • Guest agents and the fake VMM serve v2 alone, and a v1 peer is refused.
     • Fake-backed CLI tests pass through v2 without feature advertisement.
     • Capture and restore, including an exec on the returned live session, pass with v1 removed.
   • Needs: SL4.4c, in nvx-rs slimming 5.1.1: NVXC v2 carries every control path, and streams execs #1128.
   • Estimate: 0 core lines (deletions), 5 counted files.

Tests. The remaining tests, fake-backed CLI tests included, pass on v2 alone, and a v1 peer is refused. Capture and restore, with an exec on the returned session, pass with v1 gone.

Done when: every PR above has merged, with dev green.