Deduplicate rows in table reporter with identical displayed fields

[abhiojha8]

Issue number of the reported bug or feature request: #857

Describe your changes

The table reporter creates one row per AllocationRecord but only displays the top stack frame (max_stacks=1). Records with different full call stacks that map to the same displayed top frame appeared as duplicate rows in the HTML table.

This change aggregates records in TableReporter.from_snapshot() by their displayed key (tid, allocator, top_frame), summing size and n_allocations. No data is lost — the totals are preserved exactly.

Additionally, this adds a --split-threads option to the table command, matching the flamegraph reporter's existing flag. By default threads are merged; --split-threads keeps them separate.

Closes: #857

Testing performed

• All 630 tests pass (including 7 new unit tests and 1 updated integration test)
• Verified on the exercise_3/lru_cache.py tutorial: 21 leaked allocation records that previously produced 21 rows (with 18 visual duplicates) now correctly produce 3 deduplicated rows with identical size/allocation totals
• Pre-commit hooks all pass (ruff, black, isort, prettier, rstcheck, etc.)

[pablogsal]

Something is very wrong here ....

[abhiojha8]

I force-pushed a cleaned version of this branch. The earlier oversized diff was caused by CRLF line endings in a few edited files, which made GitHub show large unrelated changes. The PR diff is now back to the intended 5-file patch for the table reporter fix and the focused table reporter tests pass on the updated commit.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.48%. Comparing base (87e7f62) to head (67ae8db).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #892      +/-   ##
==========================================
- Coverage   92.52%   92.48%   -0.05%     
==========================================
  Files          99       99              
  Lines       11726    11770      +44     
  Branches      426      426              
==========================================
+ Hits        10850    10885      +35     
- Misses        876      885       +9     

Flag	Coverage Δ
cpp	92.48% <100.00%> (-0.05%)	⬇️
python_and_cython	92.48% <100.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[tvalentyn]

seeing a lint error:

src/memray/reporters/table.py:39: error: Missing type arguments for generic type "tuple" [type-arg]