Add Common Errors reference page

src/en/errors.md

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+---
+layout: base.njk
+title: Common Swift Concurrency Errors
+description: Quick explanations and fixes for frequent Swift concurrency compiler errors, with links back to the main guide.
+lang: en
+dir: ltr
+nav:
+  overview: Overview
+  async-await: Async/Await
+  sendable: Sendable
+  execution: Isolation
+  tasks: Tasks
+  keypaths: Key Paths
+footer:
+  madeWith: Made with frustration and love. Because Swift concurrency doesn't have to be confusing.
+  viewOnGitHub: View on GitHub
+---
+
+<section class="hero">
+  <div class="container">
+    <h1>Common <span class="accent">Swift Concurrency Errors</span></h1>
+    <p class="subtitle">What the compiler is yelling about and where to fix it.</p>
+    <p class="credit">Use these errors as a map back to the main guide. Every message links to the section that explains the rule.</p>
+  </div>
+</section>
+
+<section id="overview">
+  <div class="container">
+
+## [Overview](#overview)
+
+Swift's concurrency errors usually mean you crossed an isolation boundary the wrong way, forgot an `await`, or sent a value the compiler can't prove is safe. Each entry here shows the message, why it appears, and where to read more.
+
+  </div>
+</section>
+
+<section id="async-await">
+  <div class="container">
+
+## [Async/await slip-ups](#async-await)
+
+### `Expression is 'async' but is not marked with 'await'`
+
+```text
+Expression is 'async' but is not marked with 'await'
+```
+
+You're calling an `async` function but not suspending for it. The caller is either missing `await` or isn't marked `async`.
+
+- Fix: add `await`, and if the surrounding function is synchronous, move the call inside a `Task` or make the function `async`.
+- Read more: [Async/await basics](/en/#async-await).
+
+  </div>
+</section>
+
+<section id="sendable">
+  <div class="container">
+
+## [Sendable problems](#sendable)
+
+### `Sending value of non-Sendable type 'T' risks causing data races`
+
+```text
+Sending value of non-Sendable type 'T' risks causing data races
+```
+
+You moved a value across actors or Tasks that isn't `Sendable`. Swift can't guarantee the type won't be mutated from two places at once.
+
+- Fix: make the type `Sendable`, wrap shared state in an actor, or send an immutable copy that is already `Sendable`.
+- Read more: [Sendable rules](/en/#sendable).
+
+### `Cannot pass argument of non-sendable type 'T' across actors`
+
+```text
+Cannot pass argument of non-sendable type 'T' across actors
+```
+
+Similar to the previous error, but triggered when you pass a non-Sendable argument into an actor-isolated method.
+
+- Fix: add `Sendable` conformance, or restructure so the work happens inside the actor that owns the value.
+- Read more: [Sendable rules](/en/#sendable) and [Isolation basics](/en/#execution).
+
+  </div>
+</section>
+
+<section id="execution">
+  <div class="container">
+
+## [Isolation mix-ups](#execution)
+
+### `Call to main actor-isolated function cannot be made from a non-isolated context`
+
+```text
+Call to main actor-isolated function cannot be made from a non-isolated context
+```
+
+You're on a background or non-isolated function trying to call something marked `@MainActor`.
+
+- Fix: hop to the main actor with `await MainActor.run { ... }`, mark the caller `@MainActor`, or refactor so UI work stays on the main actor.
+- Read more: [Isolation domains](/en/#execution).
+
+### `Actor-isolated property 'X' can not be referenced from a non-isolated context`
+
+```text
+Actor-isolated property 'X' can not be referenced from a non-isolated context
+```
+
+Actor state must be accessed from within that actor. Reading the property directly from the outside violates isolation.
+
+- Fix: expose an `async` method on the actor, or make the caller `isolated` to the same actor so it can access the property safely.
+- Read more: [Isolation domains](/en/#execution).
+
+  </div>
+</section>
+
+<section id="tasks">
+  <div class="container">
+
+## [Task boundaries](#tasks)
+
+### `Task-isolated value of type 'T' passed as a strongly transferred parameter`
+
+```text
+Task-isolated value of type 'T' passed as a strongly transferred parameter
+```
+
+You're trying to move a value that is only safe inside the current `Task` (often non-Sendable) into another task or closure that might outlive it.
+
+- Fix: copy the needed data into a `Sendable` value before handing it off, or keep the work inside the same task instead of transferring ownership.
+- Read more: [Tasks and child tasks](/en/#tasks) and [Sendable rules](/en/#sendable).
+
+  </div>
+</section>
+
+<section id="keypaths">
+  <div class="container">
+
+## [Key path limitations](#keypaths)
+
+### `Cannot form key path to main actor-isolated property 'X'`
+
+```text
+Cannot form key path to main actor-isolated property 'X'
+```
+
+Key paths can escape to other threads, so the compiler blocks forming one to a main-actor property.
+
+- Fix: access the property directly on the main actor, or expose a computed value that is `Sendable` and not actor-isolated.
+- Read more: [Isolation domains](/en/#execution).
+
+  </div>
+</section>