refactor: modularize SnipIT development sources

[aksOps]

Summary

• make seven ordered src/*.ps1 modules and six xaml/*.xaml files the authoritative development sources
• add SnipIT.Dev.ps1 and a deterministic, transactional Build-SnipIT.ps1
• keep SnipIT.ps1 as the generated standalone zero-dependency distribution
• add complete build, parity, preflight, failure, CI, security-scope, and documentation gates
• keep Task 8 frozen and excluded

Validation

• build contract: 31 PASS labels
• pure tests: release 334/334; development 334/334
• WPF integration: release 212/212; development smoke 22/22
• deterministic release SHA-256: 5264140ECD247BA44D5BA6443C2363223775312E52181DF89E33090DBB8FA145
• AST and PSScriptAnalyzer Error: clean
• jscpd: release 0.20%; authoritative development sources 0.21%
• final independent review: ready to merge, no Critical/Important/Minor findings
• all migration commits locally signed; no Task 8 commit is an ancestor

Notes

Direct push to main is intentionally blocked by repository rules. This PR allows the eight required checks and GitHub-managed protected-branch merge path to run.