Phase 6f: text processor for .txt file ingestion

New processor: lib/processors/text_processor.py
Handles plain text files (.txt) as primary source documents.

Pipeline: acquired/text/ -> dispatcher -> text_processor.pre_flight()
-> enrich -> embed -> filing worker -> library/Domain/Subdomain/

Metadata extraction via two-source vote:
- Source A: filename parsing (title from filename)
- Source B: Gemini LLM extraction (title/author/edition/year from
  first 3 pages of text)

Page splitting reuses chunk_text() from lib/web_scraper.py.
Filing behavior matches PDFs (files to library, not organized
in-place like transcripts).

Config: adds text: text_processor to pipeline.dispatch map.
New hopper subfolder: data/acquired/text/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config.yaml

@@ -437,5 +437,6 @@ pipeline:
     pdf: pdf_processor
     stream: transcript_processor
     html: html_processor
+    text: text_processor
   # mtime stability threshold for picking up files from acquired/
   mtime_stability_seconds: 10