crawler: --fast flag exposes the verbatim AsyncWebFetcher fast_mode

The lifted AsyncWebFetcher already had a fast_mode constructor knob;
just unbroken plumbing was missing. fast_mode trades politeness for
throughput:

  - timeouts: 5s vs 15-60s
  - parallel page workers: cpu_count() * 3 vs 1
  - ignores robots.txt 'crawl-delay' directive
  - robots.txt 'Disallow' is STILL honored (only the delay drops)

Surface:

  aborist crawl --seed-url ... --fast
  make crawl-ingest URL=... FAST=1

Tests: two regressions pin that fast=True / fast=False each translate
to the right AsyncWebFetcher(fast_mode=...) construction. Capturing
fetcher fixture to avoid network. 21 bridge tests + 273 default
suite, all passing.

tests/crawler/test_bridge.py

@@ -149,6 +149,44 @@ def test_crawl_seed_dedups():
     assert len(out) == len(set(out))
 
 
+def test_crawl_seed_fast_flag_creates_fast_mode_fetcher(monkeypatch):
+    """When fast=True and no fetcher is supplied, the bridge constructs an
+    AsyncWebFetcher with fast_mode=True. Pin so the kwarg can't silently
+    drop on the floor."""
+    captured: dict = {}
+
+    class _CapturingFetcher:
+        def __init__(self, **kwargs):
+            captured.update(kwargs)
+
+        async def fetch_webpage(self, url, extract_links=False, **kw):
+            return None, []
+
+    from aborist.sources.crawler import bridge
+
+    monkeypatch.setattr(bridge, "AsyncWebFetcher", _CapturingFetcher)
+    bridge.crawl_seed("https://x.com/", max_depth=1, max_pages=1, fast=True)
+    assert captured.get("fast_mode") is True
+
+
+def test_crawl_seed_default_creates_polite_fetcher(monkeypatch):
+    """fast=False (default) → fast_mode=False on the underlying fetcher."""
+    captured: dict = {}
+
+    class _CapturingFetcher:
+        def __init__(self, **kwargs):
+            captured.update(kwargs)
+
+        async def fetch_webpage(self, url, extract_links=False, **kw):
+            return None, []
+
+    from aborist.sources.crawler import bridge
+
+    monkeypatch.setattr(bridge, "AsyncWebFetcher", _CapturingFetcher)
+    bridge.crawl_seed("https://x.com/", max_depth=1, max_pages=1)
+    assert captured.get("fast_mode") is False
+
+
 def test_crawl_seed_passes_extract_links_true():
     """Regression: AsyncWebFetcher.fetch_webpage defaults extract_links=False,
     so without the explicit kwarg BFS terminates after the seed (only one