…age views stay cacheable (#3211)

* Defer session cookie until session data is written

LifterLMS emitted the wp_llms_session_<COOKIEHASH> cookie on the first
page view of every anonymous visitor, even on pages that never touch the
session. Full-page caches won't store responses carrying a Set-Cookie
header, so this disabled anonymous page caching site-wide whenever
LifterLMS was active.

Make the session cookie lazy: a brand-new session lives in memory only
(no Set-Cookie, no DB row) until the first write via LLMS_Session::set(),
at which point the cookie is emitted. Anonymous visitors who never write
session data never receive the cookie, so those responses stay
cache-eligible. Mirrors the llms-tracking cookie approach from #2330 / #2331.

Fixes #3210

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Emit the deferred session cookie unconditionally on first write

The headers_sent() guard around the deferred set_cookie() call dropped the
cookie whenever output had already started, which broke the new behavior
under PHPUnit (and would have failed in CI). The eager cookie was
previously emitted from the constructor with no such guard, and every
first-party session write runs before output, so emit unconditionally to
match that behavior and keep it testable.

Seed a cookie in the existing-cookie session tests that previously relied
on the constructor eagerly creating one.

Verified locally: tests-run --group sessions,coupons -> 65 tests, 452 assertions, OK.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>