perf(campaign): share linear collision-safe attachment naming

Release v0.1.29. Coordinated integrity review: GovOPlaN/govoplan-core#298.

src/govoplan_campaign/backend/messages/builder.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from govoplan_campaign.backend.services.filenames import FilenameAllocator
+
 import mimetypes
 import re
 import tempfile
@@ -303,15 +305,8 @@ def _archive_filename(archive: ZipArchiveConfig, values: dict[str, Any], entry_i
     return filename if filename.lower().endswith(".zip") else f"{filename}.zip"
 
 
-def _unique_attachment_filename(filename: str, used: set[str]) -> str:
-    candidate = filename
-    path = Path(filename)
-    counter = 2
-    while candidate.casefold() in used:
-        candidate = f"{path.stem} ({counter}){path.suffix}"
-        counter += 1
-    used.add(candidate.casefold())
-    return candidate
+def _unique_attachment_filename(filename: str, used: FilenameAllocator) -> str:
+    return used.allocate(filename)
 
 
 def _deduplicated_archive_members(members: list[tuple[Path, str]]) -> list[tuple[Path, str]]:
@@ -381,8 +376,8 @@ def _attach_files(
     evidence: list[dict[str, object]] = []
     archive_members: dict[str, list[tuple[Path, str]]] = {}
     archive_attachments: dict[str, list[ResolvedAttachment]] = {}
-    used_message_filenames: set[str] = set()
-    used_zip_member_filenames: dict[str, set[str]] = {}
+    used_message_filenames = FilenameAllocator()
+    used_zip_member_filenames: dict[str, FilenameAllocator] = {}
 
     for attachment in resolution.attachments:
         attachment.message_filenames = []
@@ -394,7 +389,7 @@ def _attach_files(
             continue
         match_paths = [Path(match) for match in attachment.matches]
         if attachment.zip_enabled and attachment.zip_archive_id:
-            used_archive_names = used_zip_member_filenames.setdefault(attachment.zip_archive_id, set())
+            used_archive_names = used_zip_member_filenames.setdefault(attachment.zip_archive_id, FilenameAllocator())
             for position, path in enumerate(match_paths, start=1):
                 requested = _render_attachment_filename(
                     template=attachment.zip_entry_name_template,