toplevel: Parallelize report_package signature analysis

Parallelize the analysis of method signatures in `report_package` using
`Threads.@spawn`. Each `analyze_method_signature!` task gets its own
independent `AnalyzerState` with local `inf_cache` and `analysis_results`,
avoiding data races without complex cross-task cache sharing.

Design notes: https://publish.obsidian.md/jetls/work/JETLS/JET/JET+Make+%60report_package%60+parallelized

Key changes:
- Add `AtomicContainers` module providing thread-safe container types
  (`SWContainer`, `LWContainer`, `CASContainer`) for concurrent access
- Introduce `CASDict` for thread-safe analyzer cache (`JET_ANALYZER_CACHE`
  and `OPT_ANALYZER_CACHE`)
- Add `PackageAnalysisProgress` struct for tracking parallel analysis
  progress with atomic counters
- Parallelize `analyze_from_definitions!` in virtualprocess.jl

Benchamrks:

> `julia --startup-file-no --threads=4,2 -e 'using JET; report_package(JET; target_modules=(JET,), sourceinfo=:compact);'`
| Approach   | Time   |
|------------|--------|
| sequential | 52.07s |
| parallel   | 17.75s |

> ``julia --startup-file=no --threads=4,2 -e 'using JET; using Pkg; Pkg.activate(; temp=true); Pkg.add("CSV"); using CSV; report_package(CSV; target_modules=(CSV,), sourceinfo=:compact);'``
| Approach   | Time   |
|------------|--------|
| sequential | 44.23s |
| parallel   | 19.57s |

Written by Claude

Author: aviatesk

CHANGELOG.md

@@ -53,6 +53,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 <!-- links end -->
 
 ## [Unreleased]
+### Changed
+- **Parallelized `report_package`**: Method signature analysis in `report_package`
+  is now parallelized using Julia's multithreading, providing significant
+  speedup on multi-core systems.
+
+  > With `--threads=4,2`
+  - Benchmark on `report_package(JET)`: 52.07s → 17.75s (~3x faster)
+  - Benchmark on `report_package(CSV)`: 44.23s → 19.57s (~2x faster)
+
 ### Internal
 - Refactored the project file to use the [`[workspace]`](https://pkgdocs.julialang.org/v1/toml-files/#The-%5Bworkspace%5D-section) for the docs/test environment of JET.
   This allows running e.g. `julia --project=./test test/runtests.jl` or