windows: fix issue #128 — converter base-path parameterization, ASCII-safe output, version probe, dependency preflight

[alexyzhou]

Addresses the four repo-level issues reported in #128 (Windows 8B fresh-install
validation by @Lexlangel). Does not close the issue: the Linux question there is
tracked on the linux-host track separately.

• Cross-check gate base path: verify_against_base() hardcoded
  <repo>/windows/models/... while the app converts into ~/.edge0/models, so
  the GUI Download→Convert path hit FileNotFoundError on end-user machines (a
  dev repo's local models/ masked it). The base GGUF is now an explicit
  --base argument passed from the converter's own --out artifact; standalone
  runs derive the conventional <dir>-gguf sibling. Missing base fails with an
  E-BASE-MISSING line pointing at --base.
• Windows console output: all non-ASCII glyphs in converter-chain print
  lines (→ ⇒ ✔ × ·) replaced with ASCII, and the five entry points
  reconfigure(sys.stdout, utf-8/backslashreplace) as a guard. The app pipes the
  converter's stdout, where Python falls back to the ANSI code page — U+2192 in
  the final DONE line raised UnicodeEncodeError after an otherwise successful
  conversion.
• Engine version probe: failures were cached in a OnceLock<Option>, so one
  early miss (status poll before the engine landed on a fresh install) haunted
  the whole session with "exists but version probe failed". Only successes are
  cached now; doctor uses an always-measure server_version_probe() that
  captures stdout and stderr (the engine logs its version banner to the log
  stream; the old stdout-only/stderr-null probe could never see it) with a
  tolerant version: line match. tests/doctor.rs adds a late-binary recovery
  case.
• Dependency preflight: the converter chain is numpy-only and stays so.
  convert_mlx_to_gguf.py now fails with E-DEP guidance when numpy is
  missing; the gate's gguf fallback import reports E-DEP too. Re the PyYAML
  report: no shipped converter file imports yaml anywhere in this repo's
  history — the error comes from the mlx_lm stack the root README /
  python/pyproject.toml recommend to Windows users without a platform gate;
  that guidance issue is tracked separately on the python side.