perf: bound peak memory & hot-path algorithms (post-0.4.11 audit)

[ZhuchkaTriplesix]

Context

Static algorithm review of main (2026-07-27, post-0.4.11 / 0.4.11-a) found remaining peak-memory and hot-path weaknesses. Several older tickets were closed after partial fixes (injectSqlLimit, MySQL rowsStream, CSV isolate, Redis Future.wait) but gaps remain.

Source: Cursor algorithm review of main @ ~9a4648e.

Problem pattern

Cap / display N rows while still materializing the full source in Dart (SQL fetch, export string, zip bytes, RPC JSON line).

Child issues

P0 — peak memory

• perf(sqlite): inject LIMIT / stop unbounded rawQuery then take(cap) #415 SQLite: unbounded rawQuery then .take(cap)
• perf(postgres): stream or clamp LIMIT — avoid full Result buffer #416 Postgres: full Result buffer despite UI cap / injectSqlLimit
• perf(export): stream DataExportService to disk (stop giant String) #417 Export: DataExportService giant String (streaming CSV unused by UI)
• perf(marketplace): stream SHA256 + zip extract (no dual full buffers) #418 Marketplace / sideload zip: full-buffer hash + decode
• perf(extensions): bound and chunk RPC query result payloads #419 Extension RPC: unbounded NDJSON query result line
• perf(sdui): virtualize SduiTreeBuilder (no full Column of expanded nodes) #420 SDUI tree: no virtualization
• perf(results): avoid double-copy compute for row→string conversion #421 Result→string via compute doubles matrix peak

P1 — UI / I/O latency

• perf(mysql): offload table browse row→string conversion from UI isolate #422 MySQL table browse: sync convert on UI isolate
• perf(ui): 2D virtualization for VirtualResultGrid (wide columns) #423 VirtualResultGrid: columns not virtualized
• perf(editor): honor kSyntaxHighlightIsolateThreshold #424 Syntax highlight: kSyntaxHighlightIsolateThreshold unused
• perf(theme): incremental theme registry rescan (skip unchanged files) #425 Theme registry: full rescan every watcher event
• perf(redis): pipeline TYPE/TTL for SCAN batches (#187 follow-up) #426 Redis: pipeline TYPE/TTL (Performance: Sequential Network Round-Trips in Redis Key Scanner #187 follow-up)
• perf(sandbox): bound stderr carry to avoid O(n²) on huge lines #427 Sandbox stderr: O(n²) risk on huge lines
• perf(storage): SQL history index + prune without nested OFFSET delete #428 SQL history: prune/index shape

Already OK (do not reopen)

• MySQL SQL workspace rowsStream + client cap
• Updater streaming SHA256
• writeResultGridCsv helper (needs UI wiring → perf(export): stream DataExportService to disk (stop giant String) #417)
• Vertical VirtualResultGrid + itemExtent
• Theme parse LRU for full QueryaTheme

Acceptance

• Child issues linked with Parent: #414
• Each P0 has measurable memory / streaming acceptance criteria