fix(context): root_context() is the main Scope's context

[EdmondDantes]

Item 2 of the tutorial bug reports. The docs were right; the code was not.

Symptom

root_context() is invisible to find() from anywhere:

$c = spawn(function () { root_context()->set('cfg', 'X'); });
await($c);

var_dump(await(spawn(fn() => current_context()->find('cfg'))));  // NULL
var_dump(await(spawn(fn() => root_context()->find('cfg'))));     // 'X' — only directly

And root_context() !== current_context() at top level, which it should be — they are two different objects.

Cause

root_context() kept its context in a global of its own — ASYNC_G(root_context) — created lazily by the function itself and never attached to any Scope.

find() resolves a key by walking context->scope->parent_scope and checking each Scope's context. A context that belongs to no Scope is therefore unreachable by construction: whatever you write to it is readable only by calling root_context() again.

So there were two parallel roots — the main Scope's context (which coroutines do see) and this global (which nothing sees).

Fix

The root context is the main Scope's context — which is what the docblock already said ("Returns the root Scope"). Making it so, everything falls out of the Scope tree with no extra machinery:

• root_context() === current_context() at top level;
• every Scope chain terminates at the main Scope (scope.c gives a parentless Scope MAIN_SCOPE as parent), so find() reaches it from any coroutine that inherits from it.

The ASYNC_G(root_context) global is removed rather than kept in sync, so there is no second place for a root context to live.

Semantics, verified

root_context() === current_context() at top level	true
plain coroutine, find('cfg')	'X'
Scope::inherit(), find('cfg')	'X'
new Scope(), find('cfg')	NULL
new Scope(), root_context()->find('cfg')	'X'

A plain new Scope() is a detached root on purpose (async_new_scope(NULL, …)) — that is the whole reason Scope::inherit() exists as a separate API — so its coroutines do not inherit the context either. They still reach the root by calling root_context() directly. The stub docblocks are updated to say this precisely instead of "visible from any context", which was the overstatement in the docs.

Tests

New tests/context/010-root_context_is_main_scope.phpt pins all five rows above.

Full ext/async/tests: no new failures. The 3 that fail (curl/063, curl/064, io/082) fail identically on a pristine baseline.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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