Add tag() action for CollectionService tags

Adds a built-in `vide.tag()` action that wraps `Instance:AddTag()` /
`Instance:RemoveTag()` so tags can be applied through `create()` props.

It accepts a static string or array of strings, or a reactive source
returning either. The reactive variant diffs the previous and new tag
sets so only the delta is applied. All tags added by the action are
removed when the surrounding scope is destroyed.

Includes mock support for `AddTag`/`RemoveTag`/`HasTag`/`GetTags`,
tests, API reference docs, and a closing note in the actions tutorial.

Co-authored-by: Cursor <cursoragent@cursor.com>

test/tests.luau

@@ -24,6 +24,7 @@ local vide = require "../../vide"
     local spring = vide.spring
     local action = vide.action
     local changed = vide.changed
+    local tag = vide.tag
     local apply = vide.apply
     local step = vide.step
 local graph = require "../../vide/src/graph"
@@ -2340,6 +2341,122 @@ TEST("changed()", wrap_root(function()
     end
 end))
 
+TEST("tag()", wrap_root(function()
+    do CASE "apply single tag"
+        local frame = create "Frame" {
+            tag("MyTag")
+        }
+
+        CHECK(frame:HasTag("MyTag"))
+        CHECK(#frame:GetTags() == 1)
+    end
+
+    do CASE "apply multiple tags"
+        local frame = create "Frame" {
+            tag({ "A", "B", "C" })
+        }
+
+        CHECK(frame:HasTag("A"))
+        CHECK(frame:HasTag("B"))
+        CHECK(frame:HasTag("C"))
+        CHECK(#frame:GetTags() == 3)
+    end
+
+    do CASE "compose multiple tag() actions"
+        local frame = create "Frame" {
+            tag("A"),
+            tag("B")
+        }
+
+        CHECK(frame:HasTag("A"))
+        CHECK(frame:HasTag("B"))
+    end
+
+    do CASE "remove tags on scope destroy"
+        local _, frame, destroy = root(function(destroy)
+            return create "Frame" {
+                tag({ "A", "B" })
+            }, destroy
+        end)
+
+        CHECK(frame:HasTag("A"))
+        CHECK(frame:HasTag("B"))
+
+        destroy()
+
+        CHECK(not frame:HasTag("A"))
+        CHECK(not frame:HasTag("B"))
+    end
+
+    do CASE "reactive single tag"
+        local current = source("A")
+
+        local frame = create "Frame" {
+            tag(current)
+        }
+
+        CHECK(frame:HasTag("A"))
+        CHECK(#frame:GetTags() == 1)
+
+        current("B")
+
+        CHECK(not frame:HasTag("A"))
+        CHECK(frame:HasTag("B"))
+        CHECK(#frame:GetTags() == 1)
+    end
+
+    do CASE "reactive multiple tags"
+        local current = source({ "A", "B" })
+
+        local frame = create "Frame" {
+            tag(current)
+        }
+
+        CHECK(frame:HasTag("A"))
+        CHECK(frame:HasTag("B"))
+        CHECK(#frame:GetTags() == 2)
+
+        -- preserve "A", remove "B", add "C"
+        current({ "A", "C" })
+
+        CHECK(frame:HasTag("A"))
+        CHECK(not frame:HasTag("B"))
+        CHECK(frame:HasTag("C"))
+        CHECK(#frame:GetTags() == 2)
+    end
+
+    do CASE "reactive tag removed on scope destroy"
+        local _, frame, destroy = root(function(destroy)
+            local current = source("A")
+            return create "Frame" {
+                tag(current)
+            }, destroy
+        end)
+
+        CHECK(frame:HasTag("A"))
+
+        destroy()
+
+        CHECK(not frame:HasTag("A"))
+        CHECK(#frame:GetTags() == 0)
+    end
+
+    do CASE "reactive tag derived from source"
+        local name = source("hello")
+
+        local frame = create "Frame" {
+            tag(function() return "prefix-" .. name() end)
+        }
+
+        CHECK(frame:HasTag("prefix-hello"))
+
+        name("world")
+
+        CHECK(not frame:HasTag("prefix-hello"))
+        CHECK(frame:HasTag("prefix-world"))
+    end
+end))
+
 TEST("batch()", wrap_root(function()
     do CASE "evaluation deferred"
         local a = source(0)