progress: barra di avanzamento per download ed estrazione

Aggiunge src/progress.{c,h}: barra a una riga attiva solo quando stdout e' un
terminale (con output rediretto/pipeline non stampa nulla, quindi l'output resta
identico al client Python: parita' 30/30 invariata).

- due fasi: "Scaricamento" (byte da Content-Length) ed "Estrazione" (byte
  compressi consumati + membri processati: nuovo contatore in gz_reader e
  callback tar_progress_fn/tar_extract_cb)
- controllo con CELLAR_PROGRESS=auto|bar|plain|off (default auto)
- nessun codice ANSI, solo '\r' e riempimento con spazi; glifi ASCII se la
  locale non e' UTF-8, larghezza da TIOCGWINSZ e misurata in colonne (i glifi
  UTF-8 sono multi-byte), ridisegno throttled, velocita' a media mobile, ETA
- percorsi d'errore: progress_abort() chiude la riga senza riepilogo; il file
  parziale resta come nel client Python
- tests/progress_test.sh: pipe silenziosa, PTY via script (barra, ETA, riepilogo,
  nessun ANSI, righe entro la larghezza), modalita' plain e off, integrita' del
  file scaricato e della bottiglia installata (13/13 verdi)
- tests/parity_test.sh resta 30/30; mock_server.py con --throttle per i test

tests/mock_server.py

@@ -19,6 +19,7 @@ import json
 import os
 import re
 import threading
+import time
 from datetime import datetime, timezone
 from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
 from pathlib import Path
@@ -75,8 +76,14 @@ def parse_multipart(body: bytes, content_type: str):
 
 class Handler(BaseHTTPRequestHandler):
     state: State
+    throttle = 0  # byte/s, 0 = nessun limite (per testare la barra di avanzamento)
     protocol_version = "HTTP/1.1"
 
+    @classmethod
+    def _pace(cls, nbytes: int) -> None:
+        if cls.throttle > 0:
+            time.sleep(nbytes / cls.throttle)
+
     def log_message(self, *args):  # silenzioso
         pass
 
@@ -90,7 +97,20 @@ class Handler(BaseHTTPRequestHandler):
 
     def _body(self) -> bytes:
         length = int(self.headers.get("Content-Length") or 0)
-        return self.rfile.read(length) if length else b""
+        if not length:
+            return b""
+        if Handler.throttle <= 0:
+            return self.rfile.read(length)
+        chunks, left = [], length
+        while left > 0:
+            n = min(left, 65536)
+            data = self.rfile.read(n)
+            if not data:
+                break
+            chunks.append(data)
+            left -= len(data)
+            Handler._pace(len(data))
+        return b"".join(chunks)
 
     def do_GET(self):
         path = self.path.split("?")[0]
@@ -113,13 +133,19 @@ class Handler(BaseHTTPRequestHandler):
                     f = self.state.storage / r["stored_name"]
                     if not f.exists():
                         return self._json(404, {"detail": "Stored file not found."})
-                    data = f.read_bytes()
                     self.send_response(200)
                     self.send_header("Content-Type", r.get("content_type") or "application/octet-stream")
                     self.send_header("Content-Disposition", f'attachment; filename="{r["file_name"]}"')
-                    self.send_header("Content-Length", str(len(data)))
+                    self.send_header("Content-Length", str(f.stat().st_size))
                     self.end_headers()
-                    return self.wfile.write(data)
+                    with f.open("rb") as fh:
+                        while True:
+                            chunk = fh.read(65536)
+                            if not chunk:
+                                break
+                            self.wfile.write(chunk)
+                            Handler._pace(len(chunk))
+                    return None
             return self._json(404, {"detail": "Archive not found."})
         if path == "/openapi.json":
             return self._json(200, {"info": {"title": "Bottle Archive Server", "version": "0.1.0"}})
@@ -181,8 +207,10 @@ def main():
     ap = argparse.ArgumentParser()
     ap.add_argument("--port", type=int, default=18099)
     ap.add_argument("--state", required=True)
+    ap.add_argument("--throttle", type=int, default=0, help="byte/s massimi (per testare la barra)")
     args = ap.parse_args()
     Handler.state = State(Path(args.state))
+    Handler.throttle = args.throttle
     srv = ThreadingHTTPServer(("127.0.0.1", args.port), Handler)
     srv.serve_forever()