fix: CMYK image drop, empty-doc crash, page_index shadowing, sqlite hardening, flaky tests

Addresses items 9-13 and a/b/c/f from the max-effort review of PR #272.

- pdf.py: image colorspace check was `pix.n > 4`, which treats CMYK-without-
  alpha (n==4, same as RGBA) as not needing RGB conversion; pix.save() as .png
  then raises "unsupported colorspace", silently dropped by the surrounding
  except. Fixed to `pix.n - pix.alpha >= 4` (correctly converts CMYK, leaves
  RGBA untouched).

- pipeline.py: detect_strategy([]) (an empty/whitespace-only source file)
  returned "content_based", routing into the PDF-oriented TOC-detection
  pipeline -- wasting a real LLM call before raising IndexingError. Empty
  node lists now route to level_based, whose build_tree_from_levels([])
  returns an empty structure instantly with zero LLM calls.

- page_index.py (shim): pageindex/__init__.py binds the canonical `page_index`
  function as the package attribute, but this file is ALSO a real submodule
  of the same name -- importing it anywhere (import machinery, unconditional)
  overwrites that attribute with the module object, breaking
  `from pageindex import page_index; page_index(x)` for the rest of the
  process. Made the shim module itself callable (delegates to the real
  function via a ModuleType subclass), so whichever object ends up in that
  slot is callable regardless of import order.

- storage/sqlite.py: create_collection let a raw sqlite3.IntegrityError escape
  on a duplicate name (new CollectionAlreadyExistsError); the collections
  table's CHECK constraint only validated the name's first character (GLOB
  '*' is a wildcard, not a regex quantifier over the preceding class) --
  fixed to validate the whole string, and SQLiteStorage now also validates in
  Python (it's a public StorageEngine usable directly, bypassing
  LocalBackend's own check).

- tests/test_review_fixes_2.py: two tests used a ContentNode with no `level`
  set, so build_index took the content_based path and made real (retried,
  slow, and -- with a valid key -- billable) LLM calls instead of testing the
  text-stripping logic they claimed to. Mocked out _content_based_pipeline.

- retrieve.py: _parse_pages/_get_pdf_page_content were independent copies of
  the canonical parse_pages/get_pdf_page_content that had already drifted
  (missing the p>=1 filter and 1000-page DoS cap) -- delegate to canonical
  now, so the legacy pageindex.get_page_content path can't silently regress
  again.

- parser/markdown.py: a leading UTF-8 BOM broke first-header detection
  (not whitespace, .strip() doesn't remove it) -- decode utf-8-sig. Only
  backtick fences were recognized as code blocks, so a '#'-prefixed line
  inside a ~~~-fenced block (valid CommonMark) was misparsed as a heading --
  recognize both fence styles.

- run_pageindex.py: --if-thinning wasn't migrated to the bare-flag +
  legacy-yes/no convention the other four --if-add-* flags got; bare usage
  raised an argparse error and it never went through the shared coercion.

- types.py: DocumentDetail's `structure` field was inside the class's
  total=False body, so TypedDict rules made it optional even though every
  backend always populates it. Split into a required base class.

Adds regression tests for all of the above. Full suite: 244 passed, 2 skipped
(one pre-existing, unrelated flaky cloud-streaming test).

Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS

pageindex/storage/sqlite.py

@@ -1,8 +1,22 @@
 import json
+import re
 import sqlite3
 import threading
 from pathlib import Path
 
+from ..errors import CollectionAlreadyExistsError, PageIndexError
+
+# Mirrors LocalBackend's own collection-name rule. SQLiteStorage enforces this
+# itself (not just relying on LocalBackend's pre-check or the schema's CHECK
+# constraint below) because it's a public StorageEngine that can be used
+# directly, bypassing LocalBackend entirely.
+_COLLECTION_NAME_RE = re.compile(r'^[a-zA-Z0-9_-]{1,128}$')
+
+
+def _validate_collection_name(name: str) -> None:
+    if not _COLLECTION_NAME_RE.match(name):
+        raise PageIndexError(f"Invalid collection name: {name!r}. Must be 1-128 chars of [a-zA-Z0-9_-].")
+
 
 class SQLiteStorage:
     def __init__(self, db_path: str):
@@ -48,7 +62,17 @@ class SQLiteStorage:
         conn.execute("PRAGMA user_version = 1")
         conn.executescript("""
             CREATE TABLE IF NOT EXISTS collections (
-                name TEXT PRIMARY KEY CHECK(length(name) <= 128 AND name GLOB '[a-zA-Z0-9_-]*'),
+                -- GLOB '*' is "any characters", not a regex quantifier over the
+                -- preceding class — '[a-zA-Z0-9_-]*' alone only constrains the
+                -- FIRST character. The second GLOB (NOT ... '*[^...]*') checks
+                -- every remaining character too, so this is real defense-in-depth
+                -- for direct SQLiteStorage use (bypassing _validate_collection_name
+                -- above), not just a first-character gate.
+                name TEXT PRIMARY KEY CHECK(
+                    length(name) BETWEEN 1 AND 128
+                    AND name GLOB '[a-zA-Z0-9_-]*'
+                    AND name NOT GLOB '*[^a-zA-Z0-9_-]*'
+                ),
                 created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
             );
             CREATE TABLE IF NOT EXISTS documents (
@@ -70,12 +94,17 @@ class SQLiteStorage:
         conn.commit()
 
     def create_collection(self, name: str) -> None:
+        _validate_collection_name(name)
         with self._write_lock:
             conn = self._get_conn()
-            conn.execute("INSERT INTO collections (name) VALUES (?)", (name,))
+            try:
+                conn.execute("INSERT INTO collections (name) VALUES (?)", (name,))
+            except sqlite3.IntegrityError as e:
+                raise CollectionAlreadyExistsError(f"Collection '{name}' already exists") from e
             conn.commit()
 
     def get_or_create_collection(self, name: str) -> None:
+        _validate_collection_name(name)
         with self._write_lock:
             conn = self._get_conn()
             conn.execute("INSERT OR IGNORE INTO collections (name) VALUES (?)", (name,))