Say that any $_ENV write in worker mode outlives its request - #366

Problem

docs/php/superglobals.md described worker-mode $_ENV pinning from one side only — "writes your bootstrap makes there stay visible to every request that worker serves", and, a paragraph later, "$_ENV holds the process values plus whatever your bootstrap added". Both sentences are true, and both read as though the bootstrap is what makes a write survive.

Nothing distinguishes a bootstrap write from any other. Pinning the array is the whole of the mechanism, so $_ENV['tenant_id'] = $request->tenant(); inside a request handler stays there for the rest of that worker's life: every later request that worker serves reads it, and so does a request the worker picks up while the writer is parked at a suspension point, because $_ENV is deliberately left out of the state a suspended request carries with it. None of it is reported — no warning, no log line, only a request reading a value some other request set.

The behaviour is intentional and is not changed here; what was missing was the second half of its description. The fiber page meanwhile asserted the opposite — "Each fiber's PHP state — superglobals, response headers, output buffers, VM stack — is saved on suspension and restored on resumption. Fibers are fully isolated from each other" — on the very page the $_ENV note sends readers to for the list of suspension points.

What changed

• docs/php/superglobals.md gains a note saying that $_ENV in worker mode is worker state rather than request state, covering both the later-request and the multiplexed-neighbour case, and then says where a per-request value belongs instead: $_SERVER, which is rebuilt per request and travels with its own request across a suspension; whatever your framework scopes to the request, with the condition that disqualifies one — a container binding or a request stack set up once during bootstrap is worker state, with the same reach as $_ENV; and the request object's attributes container, for as long as that object is passed along, because oxphp_http_request() builds a fresh Request, and a fresh empty container with it, on every call.
• docs/features/fiber-multiplexing.md now lists the six superglobals the scheduler actually parks per fiber ($_GET, $_POST, $_COOKIE, $_SERVER, $_FILES, $_REQUEST) instead of naming the class, says that what is not on that list is shared while two requests overlap on a thread, and states the $_ENV carve-out. Enumerating rather than quantifying is deliberate: $_ENV is not the only PHP state outside the parked set, so any "the one exception" phrasing would have been false in a different direction.
• docs/features/worker-mode.md gains a $_ENV bullet in What Persists — the list an operator reads as the inventory of surviving state, and the one place $_ENV was absent from — and its "Between requests" reset list now names the setting the carve-out has always depended on, PHP's default auto_globals_jit=1.

No behaviour changes — documentation only.

Verification

• scripts/gen-llms-txt.sh --check — regenerated and up to date (58 pages).
• scripts/check-links.sh — 641 in-repo links, none broken, the new cross-page anchors included.
• cargo fmt -- --check, cargo clippy --no-default-features -- -D warnings, cargo test --no-default-features — clean. No Rust or C source is touched on this branch.
• The sequential half of what the note describes is already held by existing tests: a worker test writes $_ENV from inside a request handler and requires the value back on a later request, and the filter_input test asserts the same from the other side.
• The upstream fact the neighbouring filter_input(INPUT_ENV, …) note rests on was re-read at both supported PHP versions: php_auto_globals_create_env() leaves the $_ENV symbol-table entry and the engine's own copy sharing one array, so copy-on-write separates them at the first userland write — whoever makes it. That is why the filter extension keeps returning environment values after a handler has written to $_ENV, and the note now says so without attributing the write to the bootstrap.