docs: add meaningful change threshold guide

[CleanDev-Fix]

Closes #545

Summary

Adds an outcome- and evidence-based meaningful-change guide so contributors
and reviewers can distinguish small complete changes from small incomplete
changes without treating line count as an acceptance criterion.

Changes

• add docs/meaningful-change-threshold.md with complete/incomplete examples,
  insufficient-change examples, and a five-step reviewer assessment flow;
• link the guide from the README documentation index and project tree; and
• cross-link it from the existing contribution quality gate while keeping that
  gate as the source of truth for contract-safety and verification standards.

Contract Functions Changed

None. This is a documentation-only change.

Tests Added or Updated

None. No executable or contract behavior changed.

Security Considerations

No security impact. The patch changes documentation only and does not alter
balances, authorization, storage, events, external calls, dependencies, or
workflows.

Testing / Verification

• git diff --check (line-ending conversion warnings only; no whitespace
  errors)
• focused content assertions for every issue acceptance criterion
• repository-relative Markdown link validation across all three affected files
• independent QA review: PASS, with no blocking or actionable findings

The full Rust/contract suite was not run because this patch changes Markdown
only. Hosted checks are still required before this draft is marked ready.

Traceability Table

Criterion	Evidence	Status
Meaningful change threshold guide is added	docs/meaningful-change-threshold.md	Met
Line count alone is not the only measure	Guide introduction and "What makes a change meaningful"	Met
Small complete and small incomplete changes are distinguished	Four-row comparison table	Met
Insufficient examples are included	Seven issue-specific examples	Met
Reviewer assessment guidance is included	Five-step reviewer assessment sequence	Met
README links to the guide	Project tree and Documentation list	Met

[El-swaggerito]

\nThis PR is currently a draft.\n\nThe auto-merge automation will not merge draft pull requests.