Merge search and net screens into explore; drop both commands.

`explore` was already a superset of `search` (4 columns: module → type
→ filtered children → detail, with parts/signals/connections — vs
search's 2 columns of parts/signals only). It now also subsumes the
former `net` screen: when a signal entry is selected, the detail pane
shows the local pins followed by a `Net members (across connections)`
section listing every `(module, signal, type)` reachable through the
BFS over `Connection::pin_map`, with the count + dominant type and an
INCONSISTENT flag in the signal-detail header.

Removed:
- `src/tui/screen_search.cpp`, `src/tui/screen_net.cpp`.
- `commands["search"]`, `commands["net"]` (including its textual
  inline form). The `find_net` / `Net` API stays for explore's BFS
  panel and the analyze screen's net-mix check.
- `[s]` and `[n]` letter shortcuts on the dashboard.
- `net_*` and `search_*` state members + builders + constructor
  inits.

screen_idx renumbering (the slots vacated by search + net are
removed, not left dead):
  0 = console  (unchanged)
  1 = connect
  2 = set-connector-type
  3 = explore  (unchanged number, but now subsumes search + net)
  4 = dashboard (boot)
  5 = analyze

Palette signal items now jump to `explore` prefilled on the signals
tab with the child filter seeded to the exact signal name; the BFS
section in the detail pane is what shows the cross-module net.

Net-member rows in the detail pane are deliberately read-only for
now (Enter is a no-op): the signal-type popup is scoped to the
currently selected module, so opening it on a peer-module member
would mis-fire. Cross-module Enter navigation can come later if
needed.

DESIGN.md and user docs updated accordingly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

doc/user/index.md

@@ -65,8 +65,8 @@ share the same conventions:
   a sourced script. A sourced script must use the inline form of these
   commands instead.
 
-Today's interactive screens: `connect`, `search`, `set-connector-type`,
-`explore`, `net`. See [`commands.md`](commands.md) for each.
+Today's interactive screens: `connect`, `set-connector-type`,
+`explore`, `analyze`. See [`commands.md`](commands.md) for each.
 
 ## Saving, restoring, replaying