feat(search): add shared PUBLIC_FIELDS table

Create the shared field-definition table consumed by the schema builder
(_schema.py) and the whoosh-compat field registry (_registry.py). This
eliminates drift between what the index exposes and what queries can address.

- Create PublicField frozen dataclass with field metadata
- Define PUBLIC_FIELDS tuple with 16 searchable fields
- Add comprehensive test suite covering field properties

The whoosh-compat pyproject.toml dependency addition is added in a
follow-up commit, with the correct [tantivy] extra, source comment, and a
matching uv.lock update.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/documents/search/_fields.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from whoosh_compat import FieldKind
+
+
+@dataclass(frozen=True, slots=True)
+class PublicField:
+    """One query-syntax-addressable field, shared by the Tantivy schema
+    builder (_schema.py) and the whoosh-compat FieldRegistry (_registry.py).
+
+    Internal-only schema fields with no query-syntax meaning of their own
+    (sort shadow fields, bigram CJK fields, simple_title/simple_content,
+    autocomplete_word, notes_text) are NOT represented here — they stay
+    hardcoded in _schema.py's build_schema().
+    """
+
+    name: str
+    kind: FieldKind
+    aliases: tuple[str, ...] = ()
+    comma_values: bool = False
+    date_only: bool = False
+    fast: bool = False
+    subpaths: tuple[str, ...] = ()  # JSON kind only
+
+
+PUBLIC_FIELDS: tuple[PublicField, ...] = (
+    PublicField("title", FieldKind.TEXT),
+    PublicField("content", FieldKind.TEXT),
+    PublicField("correspondent", FieldKind.TEXT),
+    PublicField("document_type", FieldKind.TEXT, aliases=("type",)),
+    PublicField("storage_path", FieldKind.TEXT, aliases=("path",)),
+    PublicField("original_filename", FieldKind.TEXT),
+    PublicField("tag", FieldKind.TEXT, comma_values=True),
+    PublicField("checksum", FieldKind.KEYWORD),
+    PublicField("asn", FieldKind.U64, fast=True),
+    PublicField("page_count", FieldKind.U64, fast=True),
+    PublicField("num_notes", FieldKind.U64, fast=True),
+    PublicField("created", FieldKind.DATE, date_only=True, fast=True),
+    PublicField("modified", FieldKind.DATETIME, fast=True),
+    PublicField("added", FieldKind.DATETIME, fast=True),
+    PublicField("notes", FieldKind.JSON, subpaths=("user", "note")),
+    PublicField("custom_fields", FieldKind.JSON, subpaths=("name", "value")),
+)