#193)

## What
Builds feedback-o-tron *for real* (the follow-up you green-lit),
replacing the "Grade D Alpha" stub in `feedback_ffi.zig`'s
`boj_cartridge_invoke` with a live, state-machine-backed implementation,
and switching the e2e cycle to the cartridge's **actual declared
tools**.

## Key insight
The stub already dispatched the **correct** `cartridge.json` tool names
(`feedback_register_channel/start_collecting/submit/get_stats`) — it
just returned `{"status":"stub"}`. The old e2e cycle invoked tools that
**never existed** (`open_channel/summary/export/status/list_channels`),
which is why it got `{}`. So the fix is: make the FFI real, and point
the test at the real tools.

## Changes
- **`feedback_ffi.zig`** — `boj_cartridge_invoke` now backs the 4
manifest tools with the existing `fb_*` channel state machine:
- `feedback_register_channel` → `fb_register` →
`{"slot":N,"channel":C,"state":1}`
- `feedback_start_collecting` → `fb_start_collecting` →
`{"slot":N,"collecting":true,"state":2}`
- `feedback_submit` → `fb_submit` →
`{"recorded":true,"slot":N,"feedback_count":K}`
- `feedback_get_stats` → `fb_count`/`fb_positive_count`/… →
`{"total_feedback":…,"positive":…,…}`
- `channel`/`slot`/`sentiment` are read from the JSON args via a small,
`std.json`-free byte scanner (`jsonInt`) — sufficient for the flat arg
objects and version-stable.
- **`feedback_ffi.zig` test** — the in-file invoke test now drives the
real `register → start → submit ×2 → get_stats` cycle and asserts the
real shapes (so `zig build test` covers it in CI).
- **`tests/e2e_full.sh` Step 6** — rewritten to exercise the real tools
via the `"arguments"` object (the field BojRest forwards to the FFI);
asserts `slot` / `collecting:true` / `recorded:true` / `total_feedback`.
Removes the `FEEDBACK_OTRON` skip from #192.
- **No Elixir change** — the router already forwards `arguments` and
returns the FFI's parsed JSON as the body. The cross-language flow (Zig
JSON → Invoker parse → Jason re-encode → test) lands the keys the
assertions read.

## Verification — please note
`e2e_full.sh` passes `bash -n` + `shellcheck` locally. **The Zig could
not be compiled locally**: this sandbox's network is GitHub-only —
`ziglang.org` returns **403** and Zig isn't on GitHub releases, and apt
is blocked. So the FFI is verified by **CI** (`abi-drift.yml` +
`e2e.yml` both run `zig build`) and the added Zig unit test. I'll watch
this PR and fix any compile error promptly. (If you'd rather I verify
locally, allowing `ziglang.org` in the environment's network policy
would let me install Zig 0.15.1.)

The ABI is unchanged (only the dispatch body), so `abi-drift` should
stay green.

🤖 Draft via Claude Code.

---
_Generated by [Claude
Code](https://claude.ai/code/session_019tMcRS1Dm1nWjjYP4WvbJa)_

Co-authored-by: Claude <noreply@anthropic.com>