Add optional fields and variable-length field support

[Copilot]

The library had no way to express fields that are conditionally absent or whose size is determined at runtime. This adds three composable, declarative primitives that fit naturally alongside the existing field factories.

optional(descriptor, presence) — new in fields.ts

Wraps any field descriptor to return T | null when the field is absent. Two strategies:

• Sentinel: optional(u16(0), { sentinel: 0xffff }) — null when stored value equals sentinel (Object.is); writing null stores the sentinel.
• Predicate: optional(u32(4), (dv) => (dv.getUint8(0) & 0x01) !== 0) — null when predicate returns false; writing null is a no-op.

Writability of the result mirrors the wrapped descriptor: writable ↔ input has a setter, read-only ↔ input is read-only (e.g. substruct, typedArray). Enforced via TypeScript overloads.

const Packet = defineStruct({
  flags:   u8(0),
  // present only when bit 0 of flags is set
  payload: optional(u32(4), (dv) => (dv.getUint8(0) & 0x01) !== 0),
  // absent when value is 0xFFFF
  tag:     optional(u16(8), { sentinel: 0xffff }),
})

Dynamic-length string() overload — fields.ts

string(offset, { length: propertyName | (dv) => number }) — read-only string whose byte length is resolved at access time from another field or an arbitrary DataView function.

const Frame = defineStruct({
  name_len: u8(0),
  name:     string(1, { length: "name_len" }),          // via property
  // or:
  data:     string(1, { length: (dv) => dv.getUint8(0) }), // via function
})

Write support is intentionally omitted: writing a variable-length string requires coordinating the length field, which is better expressed with fromDataView.

Function-based length in typedArray() — fields.ts

The length option now additionally accepts (dv: DataView) => number, complementing the existing number | string | undefined forms. Fully backward compatible.

const Blob = defineStruct({
  count:  u8(0),
  values: typedArray(4, { species: Float32Array, length: (dv) => dv.getUint8(0) }),
})

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