refactor(core): split widget renderer by concern

[RtlZeroMemory]

Summary

• Split packages/core/src/app/widgetRenderer.ts into internal helper modules.
• Kept the public API unchanged.
• No intended behavior change.

Why

• Reduce the largest runtime renderer monolith before deeper runtime cleanup.

Validation

• npm run lint
• npm run typecheck
• npm run build
• node scripts/run-tests.mjs --filter "packages/core/dist/app/__tests__/"
• node scripts/run-tests.mjs --filter "packages/core/dist/__tests__/integration/"
• node scripts/run-tests.mjs
• npm run build:native
• node scripts/frame-audit-report.mjs /tmp/rezi-frame-audit.ndjson --latest-pid
• node scripts/frame-audit-report.mjs /tmp/rezi-frame-audit-2.ndjson --latest-pid

PTY / frame-audit evidence:

• Worker-mode audit session 1 (/tmp/rezi-frame-audit.ndjson): backend_submitted=5244, worker_payload=5244, worker_accepted=5244, worker_completed=5244, hash_mismatch_backend_vs_worker=0.
• Worker-mode audit session 2 (/tmp/rezi-frame-audit-2.ndjson): backend_submitted=6753, worker_payload=6753, worker_accepted=6753, worker_completed=6753, hash_mismatch_backend_vs_worker=0.
• Mouse click path: /tmp/starship.log shows shell.layout switching from bridge to comms at 2026-03-17T09:28:52.535Z with no preceding runtime.command go-comms, matching a direct UI interaction path.
• Overlay open path: /tmp/starship-2.log shows runtime.command comms-hail at 2026-03-17T09:36:41.687Z; the paired audit records around 2026-03-17T09:36:41.542Z -> 2026-03-17T09:36:41.562Z change the comms drawlist from byteLen=23832/cmdCount=397 to byteLen=24316/cmdCount=406 with aligned backend/worker completion.
• Command palette overlay path: /tmp/starship-2.log shows runtime.command toggle-command-palette at 2026-03-17T09:38:02.827Z; later comms drawlists reach byteLen=37464/cmdCount=616 while the audit remains fully aligned.
• Resize / layout path session 1: /tmp/starship.log records 100x32 at 2026-03-17T09:30:30.197Z (compactHeight:true, showRouteHealth:false) and 120x40 at 2026-03-17T09:30:40.664Z (compactHeight:false, showRouteHealth:true).
• Resize / layout path session 2: /tmp/starship-2.log records 100x32 at 2026-03-17T09:38:13.262Z and 120x40 at 2026-03-17T09:38:14.263Z, again with the expected shell.layout mode flip.
• The focus-traversal, text-input, and wheel/scroll subpaths were exercised manually during PTY runs. Starship's debug log does not serialize typed query contents or wheel deltas, so those paths are only indirectly reflected via resulting drawlist/hash churn rather than a dedicated app-level record.