refactor(fe): trim impl-history and duplicated-docs prose from comments

Remove or shorten comment blocks that:
- explain where code used to live (impl history)
- duplicate rationale already in docs/web-ui.md
- contain speculative/future-work notes

Changes:
- tabs.js: drop "SYST3M tab used to render this" history; keep badge purpose
- schedules.js: drop "used to be a card layout" history + speculative nit note
- stream-worker.js: replace 10-line problem-statement with docs pointer
  (rationale already in docs/web-ui.md SSE multiplexing section); trim
  future-work parenthetical from subscription-tracking comment
- system-sections.css: drop extraction-history prose; keep what it does
- core.css: drop move-history prose; keep what it imports and why

Part of issue cleanup per operator feedback.

frontend/packages/dashboard/src/system-sections.css

@@ -1,22 +1,7 @@
 /* ─── SYST3M/BU1LDS section panels — shared by dashboard, /core.html,
    and /builds.html ──────────────────────────────────────────────────
-   The rebuild queue, meta inputs, kept state, and container-load panels
-   render on /core.html; rebuild queue + meta inputs + live-log also
-   render on /builds.html (the build lifecycle hub). These
-   section-internal rules used to live in dashboard.css and were pulled
-   in by /core.html via a wholesale `@import "./dashboard.css"`, which
-   dragged the entire ~39kb dashboard stylesheet into the bundle.
-
-   They now live here and are `@import`ed by core.css, dashboard.css,
-   and builds.css. Each page gets only this small file instead of all
-   of dashboard.css; the dashboard keeps the rules (a couple are still
-   referenced from dashboard JS — `.rqe-source*` by the schedules view,
-   and `.hive-stats-table`, which already lives in common.css and is
-   loaded by all pages). Behaviour-preserving: identical rules, same
-   custom properties from colors.css / theme.css / common.css.
-
-   The selectors here are uniquely named to these panels and are not
-   redefined elsewhere, so the cascade is unaffected by the move. */
+   Rebuild queue, meta inputs, kept state, and container-load panels.
+   Imported by core.css, dashboard.css, and builds.css. */
 
 /* ─── meta inputs panel ────────────────────────────────────────────── */
 .meta-inputs {