fix(qwen): use stable --acp flag

[B-A-M-N]

Summary

Updates the Qwen adapter to use the stable --acp flag instead of the deprecated --experimental-acp flag.

Qwen Code v0.15.6 graduated --acp from --experimental-acp (PR #1355). The adapter code already used --acp, but the documentation comment referenced the vague `v0.x` and there were no tests verifying command construction.

Scope

Qwen-only compatibility update for the upstream-watch report covering Qwen Code v0.15.3 → v0.15.6.

Out of scope:

• Gemini adapter changes
• Codex/app-server changes
• Copilot adapter changes
• Cursor ACP workaround changes
• Shared ACP client refactors

Changes

• Updated Qwen adapter comment from `v0.x` → `v0.15.6` to reflect the actual upstream version
• Added `plugins/multi/scripts/test/qwen-acp-flag.mjs` — test that verifies the adapter constructs commands with `--acp` and not `--experimental-acp`
• Test includes source-code static analysis + live binary verification when Qwen is installed

Validation

Commands run:

```bash
node plugins/multi/scripts/test/qwen-acp-flag.mjs

→ All tests passed (against live Qwen Code v0.15.6)

```

Manual check:

```bash
qwen --help | grep -E "acp|experimental-acp"

→ --acp listed as stable flag, --experimental-acp no longer in help

```

Risk

Low. The adapter code already used `--acp`. This PR makes the documentation comment version-accurate and adds test coverage.

References

Addresses upstream-watch issue for Qwen Code v0.15.3 → v0.15.6.