Add practical module deepening checklist

[BLCCoreStudio]

Summary

Adds a contributor-facing checklist for upgrading existing overview modules toward guide/deep-dive maturity without turning the editorial standard into a rigid template.

The checklist covers:

• current-state and gap review before editing
• mental models and architecture boundaries
• practical workflows and recovery paths
• performance/reliability/security/privacy trade-offs
• deliberate alternatives and graph relationships
• primary-source and verification discipline
• focused learning navigation
• kind-specific review prompts
• editorial-regression checks
• repository validation expectations

This directly addresses the roadmap item for clearer templates/checklists for deepening existing modules while preserving the project rule that maturity requires factual/editorial review, not only automation.

[BLCCoreStudio]

Superseded by PR #102, which carries the checklist onto the current main baseline. Core docs already reference this checklist, so the replacement avoids retaining this stale/conflicted base while restoring the missing file.