feat(browser): Add bfcacheIntegration - #22397

Adds a dedicated `browser-bfcache` e2e app that exercises
`bfcacheMetricsIntegration` against a genuine back/forward cache: real
cross-document navigation served by `vite preview`, restored via
renderer-initiated `history.back()`, under the full Chrome-for-Testing
binary (`channel: 'chromium'`) with the disable flag stripped and
`BackForwardCache` enabled.

Covers hits, misses with real `notRestoredReasons`, and deliberately
bfcache-ineligible pages via `?botch=`: an `unload` listener (stable
blocker) and an open WebSocket (blocks only before Chrome 149, so that
assertion is version-gated). `Cache-Control: no-store` no longer blocks.

Removes the simulated browser-integration bfcache suite, which faked the
restore with a synthetic `pageshow` and a manual flush; it is superseded
by the real assertions here. The reason-parser unit test stays.

The unload miss deterministically also yields a privacy-masked reason.
Assert the integration reports it as a `masked` frame, giving real-browser
coverage of the masked-frame classification path (previously only unit-tested).

A plain open IndexedDB connection (and even an in-flight transaction) no
longer blocks bfcache in current Chrome, contrary to web.dev's list. The
condition that still blocks is a connection holding up a version upgrade,
which yields an `idbversionchangeevent` reason. Add a deterministic e2e
case for it and document how real Chrome diverges from the article.

A no-store (CCNS) page is only evicted from bfcache once a cookie changes;
neither the header nor a cookie change blocks on its own. Add a `?botch=nostore`
case that serves the document with `Cache-Control: no-store` (via a preview
middleware, since a static server can't set per-request headers) and mutates a
cookie, asserting the `response-cache-control-no-store` reason.

Also trim the version-specific blocker details from the README so they don't
drift; the botch cases and their assertions are the source of truth.

`browser.bfcache.navigation_type` was 1:1 with `browser.bfcache.outcome`
(hit <-> back-forward-cache, miss <-> back-forward), so it added a second
attribute encoding the same bit. Keep only `outcome`; navigation-type
vocabulary belongs on web-vital samples (future soft-navigation work), not
on bfcache counters. Also drops an unnecessary PageTransitionEvent cast.

Adds two real-browser e2e cases: an embedded bfcache-eligible iframe keeps
the top page restorable (hit), and an iframe with an unload listener makes
the top page ineligible with the reason reported from the child frame. This
gives the `frame: 'child'` classification real coverage (previously unit-only).

`frame` no longer special-cases the `masked` reason value; it now reflects
only the frame's position in the notRestoredReasons tree (`top`/`child`).
The `masked` value already lives on the `reason` attribute, so tagging the
frame as `masked` too was redundant. The integration no longer inspects any
reason string - reasons pass through verbatim.

…utes

Undefined attribute values are dropped downstream, so assign them
directly instead of spreading conditional objects. Use the
SENTRY_SEGMENT_NAME constant from @sentry/conventions for the route
attribute instead of a hardcoded string.

Capping the not-restored reasons by default silently dropped data, so a
page blocked for more reasons than the cap looked cleaner than it was.
Report every reason by default and treat `maxReasons` as an opt-in cap.

The platform bounds `notRestoredReasons` to a small standardized set per
navigation, so reporting all carries no realistic cardinality risk. A cap
below 1 drops everything, so clamp it to 1 and warn on debug builds.

…on name

Compute the transaction name once per pageshow and thread it through the
metric helpers instead of re-reading the scope for each metric. Extract
the not-restored reason emission into its own helper and correct the
pageshow event type.

…tion

Name the integration after what it observes, consistent with
webVitalsIntegration and the rest of the integration exports. bfcache is
the platform term, spelled lowercase as one word.

…nd miss

Covers three segment-name behaviors: a hit carries the parameterized route
still on the scope after the freeze, a hit without a routing integration
falls back to the raw pathname, and a miss reload falls back to the reloaded
page's pathname rather than any pre-freeze route.