Add ability to insert rows and columns

[vincentlaucsb]

Added CSV editing support:

• DataFrame::insert_row()
• DataFrame::insert_column() / append_column() via dump-and-reparse
• DataFrameRow::erase()
• DataFrameColumn::erase() as a soft delete logical-to-physical column mapping and sparse overlay shifting

Split the large DataFrame header:

• include/internal/data_frame.hpp is now mostly an umbrella shim
• implementation moved under include/internal/data_frame/
• added local include/internal/data_frame/AGENTS.md

Refactored parallel parsing/thread-pool internals:

• consolidated indexed task pool logic into include/internal/parallel/indexed_task_pool.hpp
• removed old ThreadPoolRefactor.md
• touched parser/speculative parallel internals and chunk offset reporting

Added Codecov agent guidance:

• new CODECOV_AGENTS.md
• root AGENTS.md references it

[codecov]

Codecov Report

❌ Patch coverage is 91.28739% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.63%. Comparing base (9d70013) to head (dcda238).

Files with missing lines	Patch %	Lines
include/internal/data_frame/data_frame.hpp	91.58%	6 Missing and 28 partials ⚠️
include/internal/data_frame/data_frame_cell.hpp	84.09%	2 Missing and 5 partials ⚠️
include/internal/speculative/parallel_parser.hpp	56.25%	0 Missing and 7 partials ⚠️
include/internal/data_frame/data_frame_column.hpp	89.79%	0 Missing and 5 partials ⚠️
include/internal/parallel/indexed_task_pool.hpp	94.20%	0 Missing and 4 partials ⚠️
include/internal/data_frame/row_overlay.hpp	93.75%	0 Missing and 3 partials ⚠️
include/internal/data_frame/data_frame_row.hpp	96.77%	0 Missing and 2 partials ⚠️
include/internal/memory/raw_csv_field.hpp	50.00%	0 Missing and 2 partials ⚠️
...nclude/internal/data_frame/data_frame_executor.hpp	94.44%	0 Missing and 1 partial ⚠️
include/internal/memory/block_arena.hpp	75.00%	0 Missing and 1 partial ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
+ Coverage   91.12%   91.63%   +0.51%     
==========================================
  Files          47       55       +8     
  Lines        3346     3527     +181     
  Branches      942     1037      +95     
==========================================
+ Hits         3049     3232     +183     
+ Misses        130      114      -16     
- Partials      167      181      +14     

Flag	Coverage Δ
cpp	91.49% <91.28%> (+0.55%)	⬆️
python	93.64% <ø> (ø)

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