feat: introduce ArgCodec system with symbol tags for built-in codecs

[Lojhan]

Summary

Replaces the ad-hoc type-switch logic in shared-resources.ts with a composable codec registry, and exposes a public API for users to register custom serialization strategies for class instances and other non-JSON-representable values.

Motivation

The previous implementation handled Date, Map, Set, undefined, BigInt, arrays, and plain objects through a series of if/else branches hardcoded in encodeArg, decodeArg, and writeBackObject/writeBackArray. This made it impossible for users to teach the system about their own types (e.g. class instances), and any new built-in type would require modifying the core file directly.

Three concrete bugs also existed:

• Class instances lost their prototype after an IPC round-trip (encoded by JSON.stringify as a plain object, decoded as a plain object — no reconstruction).
• Function-valued properties were deleted from objects during writeBack because the old code iterated all keys without distinguishing functions.
• Custom class instances passed as RPC arguments threw TypeError instead of degrading gracefully.

Changes

New public API

• ArgCodec<T> type — defines tag, is, encode, decode, and an optional writeBack hook with a reconcile callback for nested in-place updates.
• SharedResourcesConfig type — { codecs?: ArgCodec<any>[] } passed to sharedResources().
• configureCodecs(codecs) — registers codecs globally; new codecs are prepended (checked first) and merge by tag, so a later registration with the same tag replaces the earlier one.
• resource.configure({ codecs }) — per-resource codec registration that runs in both parent and child via module evaluation.

codecs — seven new files

Each built-in type is now a self-contained codec module. Tags use Symbol.for('sr:*') so they can never be accidentally overwritten by a user codec that picks the same string tag.

File	Handles	Tag
undefined.ts	undefined	Symbol.for('sr:u')
bigint.ts	bigint	Symbol.for('sr:bi')
date.ts	Date	Symbol.for('sr:d')
map.ts	Map	Symbol.for('sr:m')
set.ts	Set	Symbol.for('sr:s')
array.ts	Array	Symbol.for('sr:a')
object.ts	Plain objects + isPlainObject	Symbol.for('sr:obj')

shared-resources.ts

• encodeArg, decodeArg, decodeEncoded, tryReconcileInPlace all simplified to codec-registry loops — no more hardcoded type checks.
• tagToWire(tag) helper converts symbol tags to their Symbol.keyFor() string for the wire t field.
• decodeEncoded resolves symbol-tagged codecs by matching Symbol.keyFor(c.tag) === enc.t.
• Class instances without a registered codec are transparently encoded via objectCodec (own enumerable data properties, functions skipped) and reconciled back onto the original instance on the caller side — prototype is preserved, data is updated.
• Removed: encodeObjectValues, encodeObject, decodeObjectValues, writeBackArray, writeBackObject, writeBackDate, writeBackMap, writeBackSet, and the collision-escape (esc) mechanism (no longer needed since the outer { __sr_enc, t, v } sentinel wraps encoded data).

Tests

• New unsupported-args.test.ts — covers codec-based class instance round-trip (prototype reconstructed via pointCodec) and function-property preservation.
• Expanded encode-decode.test.ts — class instance encoding, nested class instances, function property omission, custom codec sentinel wrapping, codec inside object/array, tag deduplication, unknown-tag error message.
• Expanded write-back.test.ts — class instance in-place update, function property retention, custom writeBack hook invocation, reconcile callback for nested values.

Wire format compatibility

The wire format for arrays and plain objects has changed:

Value type	Before	After
Array	[...] (raw)	{ "__sr_enc": "c", "t": "sr:a", "v": [...] }
Plain object	{...} (raw)	{ "__sr_enc": "c", "t": "sr:obj", "v": {...} }