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>

test-cases/ec05_multiline_cells.csv

@@ -0,0 +1,8 @@
+id,address
+1,"123 Main St
+Apt 4B
+NYC NY 10001"
+2,"456 Oak Ave
+Suite 200
+LA CA 90001"
+3,"789 Pine Rd

test-cases/ec07_unicode_decomposed.csv

@@ -0,0 +1,5 @@
+name,translation
+Café,Cafe
+éclair,eclair
+你好,Hello (CN)
+שלום,Hello (HE)

test-cases/ec08_all_numeric.csv

@@ -0,0 +1,5 @@
+x,y,z
+1,1.1,10
+2,2.2,20
+3,3.3,30
+4,4.4,40

test-cases/ec09_smart_chars_full.csv

@@ -0,0 +1,6 @@
+field
+‘single curly’
+“double curly”
+low-9 ‘x’ high-reversed-9
+em — en – minus − horizontal ―
+ellipsis… narrow nbsp 

test-cases/uc16_shopify_nbsp_names.csv

@@ -0,0 +1,5 @@
+first_name,last_name,phone
+John ,Smith,555-1234
+Jane,Doe ,555-5678
+ Bob,Jones,555-9012
+Alice,Brown,555-3456

test-cases/uc17_product_smart_quotes.csv

@@ -0,0 +1,4 @@
+sku,title,description
+DOG-001,“Best Dog Collar”,High quality…
+CAT-002,Cat Toy — Premium,It’s the best
+FISH-003,Fish Food – Tropical,Use don’t overfeed

test-cases/uc18_excel_csv_utf8_bom.csv

@@ -0,0 +1,4 @@
+customer_id,name,amount
+1001,Alice,100.0
+1002,Bob,200.0
+1003,Charlie,300.0

test-cases/uc19_pasted_sku_zerowidth.csv

@@ -0,0 +1,4 @@
+sku,qty
+ABC​-123,10
+XYZ-456​,20
+QQQ-789,30

test-cases/uc20_bank_memo_crlf.csv

@@ -0,0 +1,7 @@
+date,amount,memo
+2024-01-15,-1500.0,"Payment
+Monthly recurring
+Net 30"
+2024-01-16,-250.0,Single line memo
+2024-01-17,-89.99,"Standard
+purchase"

test-cases/uc21_quickbooks_trailing_spaces.csv

@@ -0,0 +1,6 @@
+vendor,ein
+ACME Corp ,12-3456789
+ACME Corp,12-3456789
+ACME Corp  ,12-3456789
+Globex Inc,98-7654321
+Globex Inc ,98-7654321

test-cases/uc22_unicode_accents.csv

@@ -0,0 +1,4 @@
+company,city
+Café Roma,Boston
+Très Belle,Montréal
+Naïve Studios,São Paulo

test-cases/uc23_word_pasted_dashes.csv

@@ -0,0 +1,4 @@
+task,owner
+Phase 1 — Discovery,Alice
+Phase 2 — Design,Bob
+Q1 – Q2,Charlie

test-cases/uc24_survey_case_inconsistent.csv

@@ -0,0 +1,6 @@
+response_id,agreement,category
+1,YES,Tech
+2,yes,TECH
+3,Yes,tech
+4,yEs,Tech 
+5,yes, Tech

test-cases/uc25_lead_invisible_unicode.csv

@@ -0,0 +1,4 @@
+email,source
+alice​@test.com,Facebook
+bob@test‎.com,Google
+charlie@test.com,Organic

test-cases/uc26_mixed_line_endings.csv

@@ -0,0 +1,6 @@
+email,platform
+alice@a.com,FB
+"alice@a.com
+",Google
+"alice@a.com
+",Organic