Document parallel metadata fetching across SOUP parsers

[ydesgagn]

Summary

Updates docs/architecture.md to reflect the parallelized parser HTTP
fetching introduced in #320. The documentation now describes the shared
HttpClient::THREAD_COUNT constant, the parallel gem dependency added
to each parser, and the parallel metadata fetching algorithm.

Key changes:

• Document HttpClient::THREAD_COUNT (set to Etc.nprocessors) and the new etc external dependency in the HttpClient section
• Update Bundler, Gradle, NPM, PIP, SPM, and Yarn parser descriptions to note metadata is fetched in parallel via Parallel.map(..., in_threads: HttpClient::THREAD_COUNT)
• Add parallel to the External Dependencies list for each affected parser
• Add a new "Parallel Metadata Fetching Algorithm" section under critical algorithms describing purpose, location, and implementation

Types of changes

• Bugfix (fixes an issue)
• New feature (adds functionality)
• Refactoring (improves code without changing functionality)
• Breaking change (incompatible changes)
• Build or security update (updates dependencies, libraries, or security patches)
• Code style or documentation update (formatting, renaming, or documentation changes)
• Other (please describe):

Checklist

• Unit tests added to validate my fix/feature
• I have manually tested my change
• I did not add automation test. Why ?: Documentation-only change
• Database changes requiring migration with downtime or reprocessing of existing data
• The SOUP file lists the risk Level, requirements and verification reasoning associated with each library
• readme.md includes sections on introduction, installation, usage, and contributing
• docs/architecture.md includes sections on the architecture diagram, software units, software of unknown provenance, critical algorithms and risk controls related to PII and security
• Impact on PII, privacy regulations (CCPA/GDPR/PIPEDA), CIS benchmarks or security (availability/confidentiality/integrity); management must be notified