perf(object): O(1) dynamic property set + wide-object read index (#5054)

[proggeramlug]

Fixes #5054.

Problem

Building a wide dynamic object — obj[key] = v thousands of times — was quadratic. 10k props took ~12s; 100k never finished. Both the write and every subsequent read did a linear keys_array scan per key.

Fix — two halves

Write path (proxy.rs::ordinary_set_with_receiver)

Every obj[key] = v probed own_set_descriptor on the target, which ends in a linear keys scan. Added a fast path that reduces the write to the ordinary data-property store when nothing the spec walk models can apply: plain GC_TYPE_OBJECT written as its own receiver, no descriptor on this object (new OBJ_FLAG_HAS_DESCRIPTORS GcHeader bit, travels with the object on evacuation), class_id == 0, prototype exactly Object.prototype (no setPrototypeOf, no descriptor on Object.prototype), extensible, string key.

GLOBAL_DESCRIPTORS_IN_USE was unusable as the gate — it's poisoned at startup when the runtime installs attrs on builtin prototypes (RegExp etc.), so the very first dynamic write already saw it true. The per-object header flag plus a dedicated OBJECT_PROTO_DESCRIPTORS global are precise.

Read path (field_get_set.rs::js_object_get_field_by_name)

The 1024-entry inline field cache thrashes once an object has thousands of distinct keys. For keys arrays past 257 entries, build a validated key→index HashMap (keyed on keys_array id, LRU of 4), re-validated against the live slot on every hit — exactly the trust model the inline cache already uses. Misses fall through to the linear scan (the index is an accelerator, never authoritative); scan hits back-fill it. Side effect: indexed hits lift the old >65536-key → return undefined cliff.

Results

• 10k props: 12s → 0.05s. 100k build + read-all: never → 0.14s.
• 12-case edge battery byte-identical to node --experimental-strip-types: delete+rebuild, defineProperty non-writable/setter installed after a wide build, freeze, preventExtensions, symbol keys, Object.prototype setter intercepting missing-key writes, setPrototypeOf to a proto with a setter, spread/entries/JSON round-trip, shape sharing, hasOwnProperty/in.
• New unit test wide_object_index_reads_and_descriptor_writes.
• Gap suite: 207 pass / 27 fail, set-identical to a pristine-main baseline binary (0 regressions); the 27 are pre-existing/environmental.

Code-only PR — version bump + changelog left for merge time.