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

[ChristianMurphy]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

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.

[Copilot]

Pull request overview

This PR optimizes the markdown-to-mdast compile dispatch loop by reducing per-event allocations in the hot path, aiming to improve performance on large event streams.

Changes:

• Pre-allocates a single merged handler this-binding object (callContext) and mutates only sliceSerialize per event.
• Refactors the loop to reuse event local variable for cleaner access and slightly less indexing.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 2 changed files in this pull request and generated 1 comment.

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[Murderlon]

Fair comment