Add declarative optional and dynamic field layouts

[Copilot]

This extends structview's explicit-offset API with a complementary declarative layer for optional and variably sized fields. The new model keeps fixed layouts unchanged while allowing later fields to derive their presence, length, and offset from earlier fields.

• New declarative field capabilities

  • Added optional() to gate a field behind a boolean property or resolver
  • Field factories can now resolve offsets and lengths from:
    • literal numbers
    • property names
    • accessor/resolver functions

• What this enables

  • Optional fields without custom fromDataView() boilerplate
  • Variable-length fields whose size comes from another field
  • Follow-on fields whose offset depends on whether earlier fields are present

• API surface updated

  • Dynamic offsets now work across scalar fields and strings
  • Dynamic lengths now work for string(), typedArray(), substruct(), and arbitrary-width integer fields
  • Existing fixed-offset usage remains valid

• Docs and examples

  • Added README guidance showing how to express packet-style layouts declaratively

import { bool, defineStruct, optional, string, u16, u8 } from "@rotu/structview"

class Record extends defineStruct({
  has_name: bool(0),
  name_length: u8(1),
  name: optional(string(2, "name_length"), "has_name"),
  name_end: {
    get() {
      return 2 + (this.has_name ? this.name_length : 0)
    },
  },
  checksum: u16("name_end"),
}) {}

• Scope
  • This is an additive design: it does not replace the current explicit layout model
  • It introduces a small set of composable primitives for expressing non-fixed layouts in the same descriptor style as the rest of the library