fix(util): render [Object: null prototype] prefix in util.inspect (Node parity)

[proggeramlug]

Problem

util.inspect / console.log of a null-prototype object printed it as a bare {}, where Node prefixes it with [Object: null prototype]:

                            # node v26.3.0                  # perry (before)
util.inspect(Object.create(null)) → [Object: null prototype] {}   →  {}

These objects are common as option bags / lookup maps, so the missing prefix is a visible parity gap.

Fix

Detect the existing OBJ_FLAG_NULL_PROTO GC-header flag (set by js_object_alloc_null_proto) in the object formatter and introduce a name_prefix that resolves to:

• the constructor/class name when present,
• else [Object: null prototype] for a null-proto plain object,
• else nothing.

name_prefix replaces class_name_ref only at the three render sites (empty / single-line / multi-line). has_class_name (which drives the #private-field skip) still reflects a genuine class, so behavior for normal and class objects is unchanged.

Verification (vs Node v26.3.0)

• Object.create(null) → [Object: null prototype] {} ✓
• All 17 other inspect-probe cases unchanged (classes Foo { x: 1 }, Map(1) { … }, Set, circular <ref *1>, Error, Date, RegExp, [Getter], bigint, NaN/Infinity, …).
• perry-runtime formatting + inspect unit tests pass (12).

Found via a util.inspect differential sweep vs Node; relates to the console/util-formatting parity gap in CLAUDE.md and the parity roadmap (#4315). (Known remaining inspect nits, left out of scope: sparse-array holes render as NaN not <1 empty item>, and Node's quote-style heuristic / showHidden array [length].)

No version bump / changelog per maintainer instruction.

Summary by CodeRabbit

• Bug Fixes
  • Improved formatting for null-prototype plain objects so they consistently display as [Object: null prototype] across both compact and expanded output views.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: fea67c9a-e438-437a-ba93-8826ef2bcbf6

📥 Commits

Reviewing files that changed from the base of the PR and between 90a8a53 and 6023e60.

📒 Files selected for processing (1)

• crates/perry-runtime/src/builtins/formatting.rs

🚧 Files skipped from review as they are similar to previous changes (1)

• crates/perry-runtime/src/builtins/formatting.rs

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📝 Walkthrough

Walkthrough

format_object_as_json now reads the OBJ_FLAG_NULL_PROTO bit from the object's GcHeader reserved flags to compute a name_prefix. When no boxed_base is present and the object is null-proto, name_prefix becomes [Object: null prototype]. The empty-object closure and both the single-line and multi-line wrapper-selection match arms are updated to use name_prefix instead of class_name_ref.

Changes

Null-prototype object prefix rendering

Layer / File(s)	Summary
Null-prototype detection and prefix propagation crates/perry-runtime/src/builtins/formatting.rs	Reads OBJ_FLAG_NULL_PROTO from GcHeader reserved flags to produce name_prefix; updates the empty_object closure and both the single-line and multi-line wrapper-selection branches to switch on (boxed_base, name_prefix) instead of (boxed_base, class_name_ref).

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐇 A rabbit hops past a proto-less door,
No ancestor found on the prototype floor.
The header bit flips, the prefix shines bright:
[Object: null prototype] — displayed just right!
With single and multi-line wrapped up with care,
The null-proto object is finally there. ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: fixing null-prototype object formatting in util.inspect to match Node.js behavior.
Description check	✅ Passed	The description comprehensively covers the problem, solution, and verification, following most template sections. However, the Test plan checkbox section is not explicitly marked as completed.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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• Commit unit tests in branch fix/inspect-null-proto-prefix

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