Fix Windows dev loop: beforeDevCommand script + Vite IPv4 bind + test_acp_client.py stdin-flush

[seydousakho-star]

Summary

Two small, surgical fixes found while bringing goose2 up on Windows 11.
Both are no-ops on macOS/Linux — they only affect the Windows code
paths.

1. ui/goose2/justfile — just dev fails on Windows with a malformed
   Tauri --config argument. Three interacting causes in the
   beforeDevCommand.script string, fixed together because they share
   a code path:

   • cd ${PROJECT_DIR} && ... prefix — the unescaped && inside a
     JSON value gets split by Windows' bash → npm → tauri argv
     passthrough, truncating the --config JSON. Dropped; cwd already
     sets the working directory.
   • Leading exec — a bash builtin. Tauri runs beforeDevCommand via
     cmd.exe on Windows, which reports 'exec' is not recognized as an internal or external command. Harmless to drop on Unix.
   • Missing --host 127.0.0.1 — Vite 7 binds ::1 (IPv6 loopback)
     only on Windows by default, and WebView2 resolves localhost to
     IPv4 first, so the page never loads. Forcing IPv4 bind works on
     every platform.

   Also corrected cwd in the dev recipe from . to ... Tauri
   resolves beforeDevCommand.cwd relative to src-tauri/, not to the
   just-invocation dir. dev-debug already had ...

2. test_acp_client.py — times out with Failed to initialize: None
   on Windows. Three layered causes:

   • stderr=subprocess.PIPE with nobody reading it. cargo run -p goose-cli -- acp writes several KB of compile progress to stderr;
     Windows' ~4 KB pipe buffer fills, the child blocks on stderr
     write, and the stdio loop deadlocks before initialize can
     resolve. Routed to DEVNULL — the test only cares about JSON-RPC
     framing on stdout.
   • bufsize=0 combined with text=True — text mode always uses
     io.TextIOWrapper which needs at least line buffering. bufsize=0
     made stdout flushes unreliable on Windows. Changed to bufsize=1.
   • (surfaced only after the above was fixed) cp1252 consoles can't
     encode the unicode progress glyphs (✓ ✗ 📝). Forced
     sys.stdout.reconfigure(encoding="utf-8") so the suite runs
     uniformly on Windows, Linux, macOS.

   Opportunistic: added an optional GOOSE_BIN env override plus
   automatic fallback to target/debug/goose[.exe] so the test can skip
   the cargo-run path when a pre-built binary exists. Cuts wall time
   from minutes (recompile + cargo noise) to under a second.

Test plan

• Windows 11, Rust 1.92, Node 22, pnpm 10: just dev launches the
  native window, React UI renders, goose serve streams perf logs.
• Windows 11: full 6-phase test_acp_client.py suite (initialize,
  session/new, session/prompt, re-initialize after restart,
  session/load, prompt-to-loaded-session) passes end-to-end
  against target/debug/goose.exe.
• macOS: no behaviour change expected (dropping exec is harmless,
  --host 127.0.0.1 matches the default bind on Unix). Worth a
  reviewer confirmation.
• Linux: same as macOS — no behaviour change expected.

No new deps, no config surface changes, no doc changes needed. The
only semantic shift on non-Windows platforms is test_acp_client.py
preferring target/debug/goose over cargo run when the binary
exists, which is a speed improvement.