fix: preserve object identity across the host boundary (#4)

[rot1024]

Closes part of #4.

Problem

Reference identity was not preserved across the host boundary in the default (sync-on) mode:

arena.expose({ id: (x) => x });
arena.evalCode(`let foo = id({}); foo === id(foo)`); // was false

const shared = { k: 1 };
arena.expose({ get: () => shared });
arena.evalCode(`get() === get()`); // was false

Root cause

A host function's return value is disposed by the VM once consumed. The VMMap retains marshalled object handles for identity while sync is on — but the handle returned to the VM was the map's own handle, so the VM disposed it and the entry went stale. Marshalling the same value again then missed the map, created a fresh VM object (breaking ===) and orphaned the sibling handle (the leak fixed in #83 was the same root mechanism surfacing as a dispose-time abort).

Fix

Hand the VM a dup() of the handle when the VMMap retains it (prepareReturn), keeping the map's copy alive. find then returns a live handle on the next marshal, so identity holds.

• Scoped to host function returns (the consume point that disposed the map's handle).
• Gated on syncEnabled: with syncEnabled: false nothing is retained, so no dup — behaviour unchanged.

Behavioural note (decided with maintainer)

Identity retention means a host object marshalled once is identity-cached: a later host-side mutation is not picked up by re-marshalling. arena.sync(obj) remains the supported way to propagate host writes to the VM. The edge.test.ts > getter test documented the old re-marshal-each-time behaviour (which was also what leaked); it's updated to the new semantics, plus a new getter (synced) test shows live tracking via arena.sync. Its ctx.dispose() is un-commented now that the leak is gone.

Tests

New src/identity.test.ts pins identity (round-trip, repeated returns, retained refs, and the sync-off negative case). Full suite: 168 passing, typecheck + lint clean.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	97.5%	703 / 721
🔵	Statements	93.45%	785 / 840
🔵	Functions	95.18%	178 / 187
🔵	Branches	84.83%	498 / 587

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/index.ts	89.08%	82.2%	89.13%	91.77%	99-102, 188, 327, 341, 374-386, 397, 414, 554, 569-570
src/marshal/function.ts	100%	91.66%	100%	100%
src/marshal/index.ts	100%	90.9%	100%	100%

Generated in workflow #546 for commit 226aab6 by the Vitest Coverage Report Action