Object perf

[AvivDavid23]

Object memory reduction (~20–40% per object container)

• Pack IObject header 16B → 8B: {len:usize, cap:usize} → single u64 (32b len / 32b cap). Flat −8B per object.
• Hash-table index usize → u32: table slots hold item indices (≤cap < 2^32), so 4B suffices. Halves table: 10·cap → 5·cap bytes.
• Small-object no-table: objects with cap ≤ 8 store no hash table; lookups linear-scan the items array. Drops the table entirely for the common small object.
• Net: −40% on tiny objects (table elided), −20% on larger ones (table halved), applied per-container down the whole tree.

API — fallible allocation

• IObject mutators (with_capacity, insert, reserve, entry, …) now return Result<_, IJsonError>, mirroring IArray. Allocation failure and the 32-bit limit are recoverable instead of panicking.
• Extend/FromIterator/From<HashMap|BTreeMap> → TryExtend/TryFromIterator/TryFrom.
• Moved the shared TryExtend/TryFromIterator/TryCollect traits into new src/convert.rs; pub use re-exports keep existing paths (ijson::array::TryExtend) working.
• Infallible-context .unwrap()s (Clone, Drop/dealloc, layout, defrag, obj[k]=v) - they preserve the prior panic-on-OOM contract.

Compatibility

• Breaking: a single IObject is now capped at 2^32−1 (~4.29B) entries (was usize) — the tradeoff for the packed 8B header + halved table. (IArray is unaffected; it keeps its pre-existing 2^32−1 cap from its own packed header.)

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Note

High Risk
Rewrites core IObject storage, lookup, and resize behavior plus a breaking public API; incorrect hash/small-object logic would corrupt maps or lose keys.

Overview
IObject uses less memory per container by packing len/cap into an 8-byte header (was 16 bytes), storing hash-table slots as u32 instead of usize, and omitting the hash table when capacity ≤ 8 (linear scan only). Larger objects keep Robin-Hood probing but on the smaller table.

Breaking API: mutating IObject APIs (with_capacity, insert, reserve, entry, …) now return Result<_, IJsonError> like IArray, and infallible Extend/FromIterator/From<HashMap|BTreeMap> are replaced with TryExtend/TryFromIterator/TryFrom. Shared fallible iterator traits live in new src/convert.rs and are re-exported from the crate root (and still from array for compatibility). JSON/CBOR deserialize, serde serialize, and ijson! macros propagate allocation errors; internal paths that previously panicked on OOM still use .unwrap() where documented.

fuzz_object_ops is added to CI to fuzz insert/remove/get against a HashMap oracle, stressing small↔large table transitions and hash collisions.

Compatibility: max entries per object is now 2³²−1 (packed header tradeoff).

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