Add opt-in same-save merge for activity timeline (batch_uuid)

[AsmitNepali]

What

Adds an opt-in, read-side "same-save merge" to the activity timeline. When one save produces several activity rows (e.g. a native updated row plus a custom-field row), the consumer can render them as a single timeline entry, keyed by spatie's batch_uuid.

Why

A single save often emits multiple activity rows. Without merging, each shows as its own timeline entry, fragmenting what is logically one event. This lets consumers collapse them — without changing how activities are written/logged.

Changes

• ActivityLogSource
  • New opt-in state + fluent withSameBatchMerge(?string $mergedRenderer = null).
  • resolve(): when enabled, groups rows via groupByBatch() and yields one makeMergedEntry() per group; otherwise unchanged per-row behaviour.
  • groupByBatch(): groups rows sharing a non-empty batch_uuid, preserving the query's newest-first / first-seen order; rows without a batch_uuid each stay their own group.
  • makeMergedEntry(): representative base = first row with non-empty attribute_changes (else first row); properties = union of extractProperties() across the group; renderer set explicitly so RendererRegistry resolves it before event/type.
• TimelineBuilder::fromActivityLog(?int $priority = null, ?string $mergedRenderer = null) — passes through to the source only when $mergedRenderer is non-null. Default null ⇒ existing behaviour.
• Docs: new "Same-save merge" section in docs/.../1.sources.md (incl. the host-owned batch_uuid migration), README feature bullet, CHANGELOG entry.
• Tests: tests/Feature/SameBatchMergeTest.php — merge carries both payloads + renderer; different/null batch_uuid not merged; newest-first ordering preserved; default behaviour unchanged.

Backward compatibility

Fully opt-in. Default $mergedRenderer = null keeps the original per-row behaviour. All 55 existing tests pass. fromActivityLogOf() is not merged yet (follow-up).

Consumer requirement

Merge groups by spatie batch_uuid — the host app owns this column. If activity_log lacks it:

Schema::table('activity_log', function (Blueprint $table): void {
    $table->uuid('batch_uuid')->nullable()->index();
});

Suggested release: minor (v1.2.0).