feat: implement text cleaner (script 02) with CLI, GUI, and tests

Builds 02_text_cleaner.py from stub to working: character-level hygiene
for CSV/Excel inputs covering trim, whitespace collapse, smart-character
folding, Unicode NFC/NFKC, BOM strip, zero-width strip, control-char
strip, line-ending normalization, and per-column case conversion. Three
presets (minimal/excel-hygiene/paranoid) keep the buyer surface small.

- src/core/text_clean.py: pure helpers + CleanOptions/CleanResult +
  clean_dataframe with dtype-safe column selection
- src/cli_text_clean.py: Typer CLI mirroring the dedup CLI shape
  (dry-run by default, --apply writes cleaned + changes audit, JSON
  config save/load)
- src/gui/pages/2_Text_Cleaner.py: real Streamlit page with preset
  picker, advanced toggles, preview, before/after metrics, and three
  download buttons
- tests/test_text_clean.py + test_cli_text_clean.py: 92 new tests
  covering edge cases E1-E50 from the spec
- samples/messy_text.csv: demo dataset surfacing UC1, UC3, UC6, UC10
  in 10 rows
- test-cases/uc16-uc26 + ec05-ec09: per-use-case and per-edge-case
  fixtures

Docs: TECHNICAL.md §10.2 (full Tier 1/2/3 spec), DECISIONS.md v1.7
entry locking the spec, CLI-REFERENCE.md gains the text cleaner
section, README.md gains a top-level Text Cleaner block, USER-GUIDE.md
status row 02 promoted Skeleton -> Working.

200/200 tests pass.

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

samples/messy_text.csv

@@ -0,0 +1,13 @@
+customer_name,email,vendor,memo
+Alice Johnson,alice@example.com,ACME Corp ,Welcome aboard
+Bob Smith,bob@example.com,ACME Corp,Returning customer
+Charlie Brown,charlie@example.com,Globex,Net 30
+Diana Prince,diana​@example.com,Globex,VIP
+Edward Norton,ed@example.com,“Best Pet Supplies”,Order#42 - rush
+Frank Castle,frank@example.com,Stark—Industries,"Line 1
+Line 2
+Line 3"
+  grace HOPPER  ,grace@example.com,Globex,Loves long memos…
+Henry    Ford,henry@example.com,Ford  Motor,Industrial
+Iris	West,iris@example.com,S.T.A.R. Labs,Notewith-bell
+Jane Doe,jane@example.com,Acme,Standard