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>

docs/USER-GUIDE.md

@@ -63,7 +63,7 @@ If you prefer the command line, every script also ships as a CLI tool. See Secti
 | # | Script | Purpose | Status |
 |---|---|---|---|
 | 01 | `01_deduplicator.py` | Smart duplicate removal: exact match + basic fuzzy, configurable subset columns, full logs | Working |
-| 02 | `02_text_cleaner.py` | Character-level hygiene: trim leading/trailing whitespace, collapse internal multi-spaces, strip non-printable characters, Unicode normalization (smart quotes, em-dashes, accents), remove zero-width characters, BOM handling, line-ending normalization, case operations | Skeleton |
+| 02 | `02_text_cleaner.py` | Character-level hygiene: trim leading/trailing whitespace, collapse internal multi-spaces, strip non-printable characters, Unicode normalization (smart quotes, em-dashes, accents), remove zero-width characters, BOM handling, line-ending normalization, case operations | Working |
 | 03 | `03_format_standardizer.py` | Standardize dates, currencies, names, phone numbers, addresses | Skeleton |
 | 04 | `04_missing_value_handler.py` | Detect and handle missing values: disguised nulls (`N/A`, `-`, blanks, sentinel codes), imputation (mean/median/mode/forward-fill), required-field enforcement, drop-by-threshold | Skeleton |
 | 05 | `05_column_mapper_enforcer.py` | Rename columns, enforce a target schema | Skeleton |