feat: add <context> core tag

Provide a value to a tag's body content and consume it anywhere below
in the render tree via a statically resolved from= reference (tag name
or relative template path). Server-only (immutable) contexts compile to
zero client code -- enforced by an executable bundle assertion -- while
mutable providers fan changes out to consumers fine-grained, resume
subscriptions without re-rendering, resolve from client-created
branches (including providers nested in keyed loop iterations), and
support a writable := round trip.

Author: DylanPiercey

.changeset/context-core-tag.md

@@ -0,0 +1,45 @@
+---
+"marko": minor
+---
+
+Add the `<context>` core tag: a provider/consumer mechanism resolved
+through the render tree (dynamic extent), not lexical authoring structure,
+so a provider works correctly even when consumers are reached through
+passed content.
+
+```marko
+<context=settings>
+  <header/>   // sees settings
+  <main/>     // sees settings
+</context>
+```
+
+```marko
+<context/settings from="<settings-provider>"/>
+```
+
+A provider whose value is server-only (literals, module/`static` values,
+`$global`-derived expressions) contributes zero bytes to the client
+bundle: no provider import, no registration, no subscription machinery.
+
+Mutable providers (values derived from `let`/`input` state) fan changes
+out to consumers fine-grained: each consumer re-runs individually when
+the provided value changes, resumed pages reattach subscriptions without
+re-rendering, and consumers created client-side (eg inside an `<if>`
+toggled after resume) resolve the provider through its serialized branch
+entry. The bind shorthand makes a provider writable:
+
+```marko
+<let/cart=[]/>
+<context:=cart>
+  <checkout-panel/>  // may assign to its consumed `cart` variable
+</context>
+```
+
+A writable consumer's assignment invokes the provider's change handler,
+so the write lands in provider state and fans back out to every
+consumer.
+
+A template may consume its own context (`from=` resolving to the file
+itself): the consume resolves the nearest instance in the render tree,
+which enables recursive components and consume-inside-provide patterns.

agent-feedback/unclear.md

@@ -7,3 +7,9 @@ Things that were hard to understand, and what would have clarified them. Format
 `packages/runtime-tags/src/translator/util/runtime.ts:21` | 2026-07-02 | impact:low | effort:low
 
 `pureDOMFunctions` includes `_template`, `_await_promise`, `_await_content`, `_load_template`, and `_load_setup`, yet those factories have observable side effects at call time: `_template` calls `_resume(id, renderer)` (`packages/runtime-tags/src/dom/template.ts:42`) and the await/load factories call `_enable_catch()`/`enableBranches()` latches. The annotations are sound only because of a non-obvious invariant: registration is needed exactly when the value can be referenced by a serialized register id, which requires the value to be reachable in the client module graph anyway, and the enable latches are re-triggered by whichever construct survives tree-shaking. Two independent reviews flagged these as possibly-unsound; a comment on `pureDOMFunctions` stating the invariant would prevent repeated re-derivation.
+
+## Nested same-template `<context>` providers need an intervening resumable branch
+
+`packages/runtime-tags/src/dom/context.ts` (`_context_branch`) / `html/context.ts` | 2026-07-09 | impact:low | effort:high
+
+Same-template provider instances nested through content that never resumes (no state/closures between them) collide on one branch scope and hit the debug "share the same branch" error, because the caller-side `kBranchSerializeReason` set for a mutable provider's section does not thread through the content/dynamic-tag runtime guard (`_serialize_guard($scope_reason, holeIndex)`) the way `<if>`/`<for>` read it directly. A static parent-side `addSerializeReason(section.parent, true, sectionAccessor.binding)` in `finalizeReferences` was tried and does not reach the child's hole reason. Real recursive components carry per-instance state (which resumes the branch and resolves this naturally -- see the `context-self` fixture), so the gap is narrow; closing it is the cross-template serialize-reason edge the persisted-pages reach analysis owns.

cspell.json

@@ -382,7 +382,8 @@
     "vmax",
     "cqmin",
     "cqmax",
-    "whib"
+    "whib",
+    "reparenting"
   ],
   "ignoreRegExpList": [],
   "files": ["*"],

packages/runtime-class/test/taglib-lookup/fixtures/getTagsSorted/expected.json

@@ -40,6 +40,7 @@
     "col",
     "colgroup",
     "const",
+    "context",
     "data",
     "datalist",
     "dd",

packages/runtime-tags/src/__tests__/fixtures-interop/interop-basic-class-to-tags/sizes.json

@@ -2,8 +2,8 @@
   "dom": {
     "template.marko.page.mjs": {
       "total": {
-        "min": 65847,
-        "brotli": 20208
+        "min": 65835,
+        "brotli": 20207
       },
       "files": {
         "components/tags-counter.marko": 681,

packages/runtime-tags/src/__tests__/fixtures-interop/interop-class-to-tags-import/sizes.json

@@ -2,8 +2,8 @@
   "dom": {
     "template.marko.page.mjs": {
       "total": {
-        "min": 65847,
-        "brotli": 20208
+        "min": 65835,
+        "brotli": 20207
       },
       "files": {
         "components/tags-counter.marko": 681,

packages/runtime-tags/src/__tests__/fixtures-interop/interop-dynamic-tag-class-to-tags/sizes.json

@@ -2,8 +2,8 @@
   "dom": {
     "template.marko.page.mjs": {
       "total": {
-        "min": 65745,
-        "brotli": 20176
+        "min": 65733,
+        "brotli": 20160
       },
       "files": {
         "components/tags-child.marko": 509,

packages/runtime-tags/src/__tests__/fixtures-interop/interop-dynamic-tag-conditional-class-to-tags/sizes.json

@@ -2,8 +2,8 @@
   "dom": {
     "template.marko.page.mjs": {
       "total": {
-        "min": 65786,
-        "brotli": 20187
+        "min": 65774,
+        "brotli": 20200
       },
       "files": {
         "components/tags-child.marko": 509,

packages/runtime-tags/src/__tests__/fixtures-interop/interop-nested-attr-tags-class-to-tags/sizes.json

@@ -2,8 +2,8 @@
   "dom": {
     "template.marko.page.mjs": {
       "total": {
-        "min": 66152,
-        "brotli": 20390
+        "min": 66140,
+        "brotli": 20358
       },
       "files": {
         "components/tags-layout.marko": 838,

packages/runtime-tags/src/__tests__/fixtures-interop/interop-nested-class-to-tags/sizes.json

@@ -2,8 +2,8 @@
   "dom": {
     "template.marko.page.mjs": {
       "total": {
-        "min": 65824,
-        "brotli": 20220
+        "min": 65812,
+        "brotli": 20211
       },
       "files": {
         "components/tags-layout.marko": 696,