Step through search matches across rooms in an all-rooms search

[hayaksi1]

Stacked on #34008 (in-room match stepping) and design-gated under #21640. Please review
#34008 first — the diff here includes it and will reduce once it merges. Happy to align the UX
with the match-presentation design discussion before this merges.

Problem

In-room match stepping (#34008) steps through matches in the current room only — in an all-rooms
search the match stepper is disabled, even though the results span multiple rooms. There is no way
to step from a match in one room to a match in another.

Fix

Extend the stepping cursor (SearchSessionStore) to span rooms: in an all-rooms search the match
list is no longer filtered to the current room, so the up/down arrows step across every room's
matches (including matches in predecessor/upgraded rooms), switching the viewed room as needed. The
session cursor already lives outside any single RoomView, so the focused match survives the room
switch.

Tests

RoomView-test: stepping into a predecessor-room match, and enabling the stepper for an all-rooms
search so it steps across rooms.

Relates to #21640
Relates to #32341

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)