docs: commit ARCHITECTURE and STRUCTURE documentation updates (DOC-001)

[ffooll-bit]

Summary

Commits the pending ARCHITECTURE.md and STRUCTURE.md documentation updates that document the already-merged PISN features. The two files were long held out of scope (so they drifted from the code) and are now committed as-is.

Related issues

Fixes #119

Checklist

• php -l passes on all modified PHP files
• npm run build succeeds and committed assets are up to date
• php spark routes shows correct new routes (if routes changed)
• vendor/bin/php-cs-fixer fix passes (no style violations)
• vendor/bin/phpunit is green (if tests exist)
• No debug code: dd(), var_dump(), console.log(), print_r(), exit()
• All user inputs validated server-side
• All POST forms include csrf_field()
• All HTML output uses esc()
• No unrelated files changed
• CHANGELOG updated if this is a user-facing change
• Behaviour verified in the browser if UI changed (attach a screenshot if useful)

Screenshot (if applicable)

N/A — documentation-only change.

Notes for reviewers

Markdown-only change: ARCHITECTURE.md (+157) and STRUCTURE.md (+25/-19), plus the tracker docs/IMPROVEMENTS.md (DOC-001 → implemented). No user-facing behaviour change, so no CHANGELOG entry.