fix: ActivityKeyerComposer perf

[OEvgeny]

Fixes #

Changelog Entry

• Improved ActivityKeyerComposer performance for append scenarios by adding an incremental fast path that only processes newly-appended activities, in PR #5790, by @OEvgeny

Description

ActivityKeyerComposer re-iterated all activities on every render to assign stable keys. With 1000+ streaming delta activities, each push triggered a full O(n) scan, making overall complexity growing towards O(n²) if we push fast enough. This PR adds an incremental fast path so only newly-appended activities are processed per render.

This implies that activities are immutable.

Design

The useMemo now uses a two-path strategy:

1. Fast path (append-only): Compares the activities array prefix to the previous render using reference equality. When activities are only appended, only the new tail is iterated per render. Ref-held maps are mutated in-place so a single lookup covers both previous and current-iteration entries.
2. Slow path (removal/reorder): Falls back to the original full-recalculation when the prefix diverges (items removed, replaced, or reordered).

This is still iterating the whole array, but since we need only ref checks, we spend only about 10% of the previous time on this work.

Specific Changes

• Added prevActivitiesRef and prevActivityKeysStateRef refs to track previous render state
• Added common-prefix detection to identify append-only changes in O(min(prev, next))
• Added incremental fast path that processes only activities[commonPrefixLength..]
• Kept maps mutable in refs (removed Object.freeze on Map instances) so the fast path can append in-place
• Renamed shadowed activities variable to activitiesForKey in keyToActivitiesMap construction
• Preserved the original full-recalculation as the slow path fallback

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• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

[Copilot]

Pull request overview

Improves ActivityKeyerComposer key assignment performance during streaming/append-heavy updates by adding an incremental “append-only” fast path that avoids full rescans, while retaining a full recompute fallback for reorder/removal scenarios.

Changes:

• Added common-prefix detection and an append-only incremental update path to process only newly appended activities.
• Switched the internal key maps to mutable Maps held in refs to support in-place incremental updates.
• Updated the changelog with an entry describing the performance improvement.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/api/src/providers/ActivityKeyer/ActivityKeyerComposer.tsx	Adds append-only incremental keying logic with ref-held mutable maps and a slow-path full recompute fallback.
CHANGELOG.md	Documents the performance improvement in ActivityKeyerComposer.

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