WIP: joins, column transforms, JSON array support (#4, #11, #5)

[fahadsiddiqui]

Summary

Working through the open feature/test issues (#4-#12). This PR is a WIP snapshot, not ready to merge.

• feat: Join operations (inner, left, right, outer) #4 Join operations — done. Join/JoinOn, all four join types, composite keys, int/float key coercion, deterministic ordering, column-collision suffixing, benchmarked (~3ms for a 100k×10k inner join).
• feat: column derivation & transforms (Apply, Map, FillNA, DropNA) #11 Column transforms — done. Apply/Map/FillNA/DropNA with derived-column type inference and panic-safe user callbacks.
• feat: JSON array ([{...}]) read/write support #5 JSON array read/write — in progress. Tests written, jsonl.go refactored to share its decode/build logic, but json.go itself isn't written yet — the package does not currently build on this branch.
• Bug fix found along the way: SafeOperation never actually recovered panics (recover() was called from a helper, not directly by the deferred func, where Go treats it as a no-op). Fixed and tested.

Full status, what's left, and how to resume is in docs/PROGRESS.md.

Test plan

• go test ./... green as of the feat: Join operations (inner, left, right, outer) #4/feat: column derivation & transforms (Apply, Map, FillNA, DropNA) #11 work (prior to the feat: JSON array ([{...}]) read/write support #5 tests being added)
• Package currently fails to build — needs json.go implemented before tests/CI can run again
• Remaining issues (test: cover lazy.go gaps (Where, Error at 0%) #6, test: cover ops.go matcher gaps (match*, Unique, typedComparator) #7, test: cover err.go recoverFromPanic / Error paths #8 partial, feat: streaming/chunked reads for large CSV & JSONL files #9, feat: compound query expressions (AND/OR, parentheses) in Query() #10, feat: text data-visualization helpers (Table, BarChart, Histogram, Info) #12) not started