refactor: split the pack builder into its layers

generate_pack.py held six responsibilities in 4744 lines. Five move out
in dependency order, so nothing above reaches back down: destinations,
core extras, resolution with its storage tiers, the notes shipped inside
a pack, and the pack verifier. generate_pack.py keeps the build and the
command line, and re-exports the rest.

Two things the move surfaced. The offline switch was a module global the
command line assigned, which a re-export would have copied and frozen at
False; it is set through a call now and not re-exported. And a facade
placed after the entry point binds too late: importing the module worked,
running it did not, so the manifest run died on a name the tests never
exercised because tests import.

Verified against the previous revision on frozen inputs: every generated
artefact is identical, save the catalogue that embeds hashes of files
carrying a build timestamp.

scripts/artifacts.py

@@ -86,3 +86,17 @@ def artifact_lock(directory: str, exclusive: bool = True):
             yield
         finally:
             fcntl.flock(handle, fcntl.LOCK_UN)
+
+def _build_timestamp(db: dict | None = None) -> str:
+    """Timestamp for generated artifacts.
+
+    Reads the database snapshot the artifact was built from, so rebuilding
+    the same data twice yields the same value. Falls back to the clock only
+    when no database is at hand.
+    """
+    stamp = (db or {}).get("generated_at")
+    if isinstance(stamp, str) and stamp:
+        return stamp
+    from datetime import datetime, timezone
+
+    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")