perf(compile): build dispatch handler context as a stable-shape literal

The compile loop built each handler's this-binding with
Object.assign({sliceSerialize: ...}, context). Object.assign goes
through a generic copy-loop over the source's enumerable properties;
V8 cannot give that loop a stable hidden class for the merged object
because the source's shape is a runtime variable. Each event paid the
cost of a copy-iteration plus a hidden-class transition.

The fix replaces the Object.assign with an inline object literal that
lists every field explicitly. The literal has a fixed compile-time
shape so V8 keeps a single hidden class for the merged object across
every event, allocates it through a fast path, and can inline the
construction at the call site. The merged object is still allocated
per event, so the prior contract (handlers may capture `this` or
reassign top-level fields without leaking into subsequent events) is
preserved. The dispatch is otherwise unchanged.

Inputs that benefit, with multi-run median-of-medians vs the baseline
(spread in parentheses):

  10,000 short backtick code spans          -36.1%  (5.3%)
  1,000 inline links in a paragraph         -31.0%  (2.0%)
  10,000 character entity references        -29.9%  (2.2%)
  one CommonMark example                    -13.8%  (5.5%)
  CommonMark spec * 35  (~564 KB)           -10.6%  (3.6%)
  CommonMark spec * 7  (~113 KB)             -7.8%  (18.3%)
  full CommonMark spec  (~16 KB)             -7.4%  (43.5%, NOISY)

Every input that drives a non-trivial number of events through the
dispatch loop benefits, because Object.assign was the cost being
eliminated. Inputs with high node-count-per-byte (many small inline
tokens) gain the most.

Trade-offs and inputs where the gain is small or absent:

A 10,000-unmatched-asterisks input and a 256 KB Unicode-heavy input
both moved within +/- 1% of baseline. Their event count per byte is
low, so the per-event Object.assign was not a hotspot.

The pure emphasis stress inputs ('a**b' repeated 10,000 times and
similar) reported large deltas in single runs but their cross-run
spread is 44 to 53% on the baseline alone. The input shape (mostly
attentionSequence events that mostly do not match a handler) means
the per-event merge was not the cost driver. The deltas there are
noise.

Tests pass: dev + prod 1448/1448, 100% coverage maintained,
mdast-util-gfm 54/54, mdast-util-mdx 11/13. The two failing mdx tests
reproduce on upstream/main and are not introduced by this branch.

Author: ChristianMurphy

dev/lib/index.js

@@ -247,16 +247,32 @@ function compiler(options) {
 
     index = -1
 
+    // The handler this-binding receives the same fields as `context` plus a
+    // per-event `sliceSerialize`. Build the merged object as a stable-shape
+    // literal so V8 keeps a single hidden class for it and can inline the
+    // construction; the previous Object.assign({sliceSerialize: ...}, context)
+    // form created the same object but went through a copy-loop and gave V8
+    // less to optimize. A fresh object per event preserves the previous
+    // contract that handlers may capture `this` or reassign top-level fields
+    // without leaking into subsequent events.
     while (++index < events.length) {
-      const handler = config[events[index][0]]
-
-      if (own.call(handler, events[index][1].type)) {
-        handler[events[index][1].type].call(
-          Object.assign(
-            {sliceSerialize: events[index][2].sliceSerialize},
-            context
-          ),
-          events[index][1]
+      const event = events[index]
+      const handler = config[event[0]]
+
+      if (own.call(handler, event[1].type)) {
+        handler[event[1].type].call(
+          {
+            sliceSerialize: event[2].sliceSerialize,
+            stack: context.stack,
+            tokenStack: context.tokenStack,
+            config: context.config,
+            enter: context.enter,
+            exit: context.exit,
+            buffer: context.buffer,
+            resume: context.resume,
+            data: context.data
+          },
+          event[1]
         )
       }
     }