chore(fixtures): remove legacy m1-m4 xlsx fixtures and align lab with source-first layout

[hhimanshu]

Summary

• Deletes m1/, m2/, m3/, m4/ directories — the one-shot xlsx→TSV migration has already run; no conformance test reads these files
• Deletes migrate_fixtures.rs — the file itself said "run once, then delete this file"
• Moves lab/financial.tsv → lab/google_sheets/financial.tsv to mirror the source-first directory convention used by canonical fixtures
• Updates lab_conformance to scan lab/ recursively so it picks up cases under any source subdirectory (google_sheets/, excel/, etc.)
• Updates lab/README.md to document the source-first subdirectory layout

Closes #515

Test plan

• cargo nextest run -p truecalc-core --test conformance — all 19 tests pass
• lab_conformance finds and runs lab/google_sheets/financial.tsv
• No m1–m4 files or migrate_fixtures.rs remain

🤖 Generated with Claude Code

[github-actions]

Test Coverage by Category

Category	Unit Tests	Google Sheets Conformance	Property Cases	Total
Array	42	301/301 ✓	1,000 (2×500)	1,343
Database	35	175/175 ✓	3,500 (7×500)	3,710
Date	348	295/295 ✓	2,500 (5×500)	3,143
Engineering	245	841/841 ✓	5,500 (11×500)	6,586
Filter	11	20/20 ✓	4,500 (9×500)	4,531
Financial	149	182/182 ✓	2,000 (4×500)	2,331
Info	0	390/390 ✓	4,500 (9×500)	4,890
Logical	114	250/250 ✓	3,500 (7×500)	3,864
Lookup	69	361/361 ✓	1,000 (2×500)	1,430
Math	528	1,923/1,923 ✓	8,000 (16×500)	10,451
Operator	75	263/263 ✓	7,500 (15×500)	7,838
Parser	80	94/94 ✓	4,000 (8×500)	4,174
Statistical	480	2,557/2,557 ✓	5,000 (10×500)	8,037
Text	305	658/660 ⚠	4,000 (8×500)	4,965
Volatile	0	—	3,500 (7×500)	3,500
Web	29	56/56 ✓	6,000 (12×500)	6,085
Total	2,620	8,366/8,368	66,000 (132×500)	~76,988

_{Oracle: Google Sheets · ✓ = 100% passing · ⚠ = known deviation}