feat(ui): give CallParticipantSort an identity

[renefloor]

Two PRs landed a re-sort rule in StreamCallParticipants.didUpdateWidget, each asserting the opposite of the other, and each with a test:

• feat(ui): pick any participant layout from a menu #1332 (participant layout menu, on v2) compares layoutMode.sorting, so picking a layout reorders and a comparator passed inline does not.
• fix(llc,ui): stop re-emitting participant state for events that change nothing #1354 (participant throttling, on main) compares sort by identity, so a caller's new comparator reorders on the spot.

They only met when main was merged into v2, where one had to give. This PR is the resolution that satisfies both.

Supersedes the two earlier revisions of this PR. Those each traded one behaviour away; this gets all three, at the cost of a breaking type change.

The problem

StreamCallParticipants.didUpdateWidget has to answer "has the sorting changed?" — a comparator cannot answer it. A closure written inline is a new function on every build, so comparing them reorders the list every time. Comparing layoutMode.sorting instead avoids that, but then a caller replacing their own comparator is never noticed, and nothing else picks it up: Call.participantsStream is throttled and silent in a quiet call.

The change

CallParticipantSort becomes a class pairing the comparator with an identity:

StreamCallParticipants(
  sort: CallParticipantSort(byName, identity: 'by-name'),
)

Two sorts with equal identities are the same sort, whatever functions they hold — so one constructed inline on every build reorders nothing, and one with a new identity reorders on the spot. Without an identity a sort is only equal to itself, which is the old behaviour, documented as such.

The SDK's own sorts move to CallParticipantSorts (regular, speaker, pictureInPicture, livestreamOrAudioRoom), held once and identified, so layoutMode.sorting hands back the same instance for an unchanged layout.

	layout change re-sorts	caller's new sort applies	inline sort re-sorts every build
sort identity (on v2 now)	yes	yes	yes
layoutMode.sorting (rev 2)	yes	no	no
filter only (rev 1)	no	no	no
this PR	yes	yes	no

Breaking

A bare closure no longer assigns to sort. There is no dart fix for it — fix_data.yaml can rename and drop parameters but cannot wrap an expression — so call sites are migrated by hand:

- sort: (a, b) => a.name.compareTo(b.name),
+ sort: CallParticipantSort(
+   (a, b) => a.name.compareTo(b.name),
+   identity: 'by-name',
+ ),

PictureInPictureConfiguration.sort and StreamPictureInPictureUiKitView.participantSort take a CallParticipantSort too, rather than a bare Comparator, so there is one type across the API.

Why the identity has to live on the sort

A wrapper that kept CallParticipantSort = Comparator does not work. Dart implicitly tears off call when an object is assigned to a function type, and the tear-off drops the custom equality:

wrapper == wrapper      : true
torn-off == torn-off    : false   ← assigned to the Comparator typedef
same instance tear-offs : true

So the field has to hold the class, which is what makes this breaking rather than additive. An optional sortKey parameter alongside the comparator was the non-breaking alternative; it was passed over in favour of one coherent type while v2 is still open for breaking changes.

Testing

melos run analyze:error clean across the workspace. 501 tests pass in stream_video_flutter — the same count as v2, with #1332's two tests and #1354's all passing together for the first time. Goldens not run locally.

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 79.16667% with 5 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v2@cdff31d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...r/lib/src/call_participants/call_participants.dart	68.75%	5 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             v2    #1364   +/-   ##
=====================================
  Coverage      ?   35.21%           
=====================================
  Files         ?      387           
  Lines         ?    29567           
  Branches      ?        0           
=====================================
  Hits          ?    10413           
  Misses        ?    19154           
  Partials      ?        0           

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