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/core.css

@@ -1,21 +1,6 @@
 /* ─── /core.html — C0R3 page ───────────────────────────────────────
-   Standalone page for kept state (tombstones) and container load.
-   Same minimal-chrome pattern as /logs.html (a `← home` back-link +
-   a createTabStrip sub-tab strip; base tab + page-header styling comes
-   from @hive/shared via common.css, linked by core.html).
-
-   The section renderers (kept state, container load) emit the same
-   class names as before. Those section-internal rules live in
-   system-sections.css (shared with dashboard.css and builds.css, which
-   references `.rqe-source*` from the schedules view). The
-   `.hive-stats-table` the container-load table reuses lives in
-   common.css, linked directly by core.html. Importing just the section
-   rules keeps the C0R3 bundle small instead of dragging in all of
-   dashboard.css.
-
-   Rebuild queue + meta inputs have moved to /builds.html; the
-   `.rebuild-live-log-*` styles that used to live here have moved to
-   system-sections.css so builds.css can share them via @import. */
+   Kept state (tombstones) and container load. Section rules imported
+   from system-sections.css (shared with dashboard.css + builds.css). */
 @import "./system-sections.css";
 
 body.core-shell {