feat: switch default allocator from mimalloc to jemalloc

[jqnatividad]

Summary

• Restores tikv-jemallocator (jemalloc) as a feature-flagged allocator and makes it the new default, replacing mimalloc. This aligns qsv's allocator with Polars' preferred allocator.
• Both mimalloc and jemallocator features remain available, with a compile_error! guard preventing both from being enabled simultaneously.
• Bumps Polars rev to bb93ba8 (py-1.39.3).
• Uses exact match for MALLOC_CONF env var instead of prefix match.

Test plan

• cargo build --bin qsv -F all_features compiles successfully with jemalloc default
• cargo build --no-default-features --bin qsv -F all_features,mimalloc,jemallocator triggers compile_error!
• cargo clippy -F all_features passes clean
• qsv --version shows jemalloc in allocator field

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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[Copilot]

Pull request overview

Switches qsv’s default memory allocator from mimalloc to jemalloc (via tikv-jemallocator) to align with Polars’ preferred allocator, updates Polars rev, and adjusts related docs/env-var handling.

Changes:

• Makes jemallocator the default Cargo feature and wires tikv_jemallocator::Jemalloc as the global allocator.
• Updates --version / --envlist reporting paths for jemalloc and tightens MALLOC_CONF matching.
• Bumps Polars git rev and updates documentation/tests accordingly.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
tests/test_100/lesson_4.rs	Fixes expected COUNT(*) output for lesson 4 task 6.
src/util.rs	Updates allocator identification in --version and env-var listing logic for jemalloc.
src/mainlite.rs	Adds jemalloc global allocator and mutual-exclusion guard.
src/maindp.rs	Adds jemalloc global allocator and mutual-exclusion guard.
src/main.rs	Adds jemalloc global allocator and mutual-exclusion guard.
README.md	Updates allocator docs and Polars rev badge.
docs/PERFORMANCE.md	Updates allocator documentation and build examples for jemalloc default.
docs/FEATURES.md	Documents new default jemallocator feature.
docs/ENVIRONMENT_VARIABLES.md	Documents jemalloc env vars and updates --envlist description.
docs/contributor/PROJECT_TECHNICAL_OVERVIEW.md	Updates contributor overview to mention jemalloc default.
Cargo.toml	Adds tikv-jemallocator, sets default feature to jemallocator, bumps Polars rev.
Cargo.lock	Updates Polars git source and adds tikv-jemallocator packages (plus transitive updates).

[Copilot]

Pull request overview

Copilot reviewed 20 out of 21 changed files in this pull request and generated 6 comments.