fix: make anonymousId cross-subdomain reconciliation the default

[abueide]

Summary

Follow-up to #1398 (released as 1.84.2). That PR shipped resolveAnonymousIdConflicts as an opt-in, default-off flag -- a cautious first step taken before we'd confirmed how staged rollout actually works for this SDK.

Since then we traced the real delivery path end-to-end (ajs-renderer -> analytics.js-versions -> Flagon gates): which build of analytics-next a given source's CDN bundle contains is already controlled per-source, via v2projects pins and the ajs-renderer/v2rollout Flagon gate. That makes an application-level opt-in redundant -- exposure to this fix is governed by which code a source receives, not by a flag inside that code. Keeping the flag just adds a second, unnecessary gate: a source could receive the fix and still see the bug because nobody flipped an option in their load() call.

This PR removes resolveAnonymousIdConflicts and makes UniversalStorage#getConsistent the sole anonymousId resolution path -- no flag, no default to reason about.

Behavior change: no longer opt-in. Only affects sources where the cookie and localStorage already disagree (the broken case); when they agree, behavior is unchanged. The flag only existed for one release, so there's no meaningful adoption window for anyone depending on its absence.

Test plan

• Updated User unit tests to drop the flag parameter -- reconciliation is now the only path (packages/browser/src/core/user/__tests__/index.test.ts)
• Updated the Playwright e2e reproduction of Cross-domain tracking and clearing of LocalStorage #706 to drop the flag and assert convergence unconditionally (packages/browser-integration-tests/src/anonymous-id-subdomain-sync.test.ts) -- verified locally against a fresh build, including a full instrumented state-trace confirming the cookie wins the disagreement and resyncs localStorage as expected
• Full packages/browser unit suite passes (unrelated pre-existing flake in ajs-destination/Amplitude tests, confirmed by rerun)
• Full browser-integration-tests e2e suite passes
• tsc --noEmit and eslint clean on all touched files
• Bundle size actually decreased slightly (~29.96 kB, under the existing 30.1 kB limit) from removing the flag branch

[changeset-bot]

🦋 Changeset detected

Latest commit: a6dd87d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@segment/analytics-next	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.58%. Comparing base (53d0a6b) to head (4d8f329).

⚠️ Current head 4d8f329 differs from pull request most recent head a6dd87d

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1400      +/-   ##
==========================================
- Coverage   91.59%   91.58%   -0.01%     
==========================================
  Files         127      127              
  Lines        4163     4159       -4     
  Branches     1040     1038       -2     
==========================================
- Hits         3813     3809       -4     
  Misses        350      350              

Flag	Coverage Δ
browser	92.49% <100.00%> (-0.02%)	⬇️
core	90.07% <ø> (ø)
node	89.43% <ø> (ø)

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