docs: design notes for future PDF→CSV tool

New ``docs/FUTURE-TOOLS.md`` captures post-launch tool ideas with a
consistent shape — What / Why / Can we ship now / Approach / GUI
sketch / Effort / Risks / Ship criteria. Resting place for things
the new-tool freeze in ``PLAN.md`` §2.1 refuses to build but that
keep coming up.

First entry: **#10 PDF → CSV extractor** (bank statements et al.).

Key facts captured:

- **Current state**: no PDF infrastructure exists. Zero PDF
  dependencies in requirements.txt; zero PDF-touching code under
  ``src/``. The only "PDF" string in the codebase is the planned-
  output copy for the Quality Check tool, unrelated to extraction.
- **Library picks**: pdfplumber as the extraction core (BSD-3,
  no native compiler, gives coordinate-aware text), Tesseract via
  pytesseract as the OCR fallback for scanned PDFs,
  streamlit-drawable-canvas as the region-picker component.
- **GUI sketch**: user draws a header strip + a row template on a
  rendered page; the tool applies that template across N pages,
  saves the template by layout fingerprint for next month's
  statement, emits CSV.
- **Effort phased A–E**: 3–4 weeks for a text-only MVP; 6–10
  weeks for a polished version with multi-page template recall;
  +2–3 weeks if scanned-PDF OCR is required.
- **Difficulty**: medium-hard. The pieces are well-trodden; the
  combination (region selection that persists across pages and
  across documents with similar layouts) is where the engineering
  goes.
- **Ship criteria**: ≥1 paying customer + ≥3 paid or ≥5 demo
  emails asking for PDF extraction + the bookkeeper niche
  converting at least one customer first. None have fired.

Cross-references added:

- ``docs/REQUIREMENTS.md`` §11: pointer to FUTURE-TOOLS.md for
  parked tool ideas, with a one-paragraph summary of #10.
- ``docs/PLAN.md`` §2.1: notes that the freeze parks future tools
  in FUTURE-TOOLS.md and explicitly names #10 as the current
  highest-pressure entry.
- ``docs/NEXT-STEPS.md`` Phase 5 "what NOT to build" table: a new
  row for the PDF tool tied to the same ship-trigger language.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/PLAN.md

@@ -58,6 +58,14 @@ buy" into "an automatable workflow you depend on." That conversion is
 what produces retention and word-of-mouth — the only marketing channel
 that scales under the no-network/no-touch constraint.
 
+**Parked behind the freeze**: post-launch tool ideas are captured in
+`docs/FUTURE-TOOLS.md` with feasibility, GUI sketch, effort estimate,
+and ship criteria for each. Currently parked: **#10 PDF → CSV
+extractor** (bank statements et al.) — gated on a paying customer +
+≥3 paying customers or ≥5 demo emails explicitly asking for PDF
+extraction, with the bookkeeper niche converting at least one customer
+first. None of those triggers have fired yet.
+
 ### 2.2 The demo *is* the product. Make it embarrassingly good.
 
 - Three persona-tagged sample datasets, not one generic CSV: Shopify