Support Percent / URL encoding

[dipu-bd]

Add a byte-level percent-encoding codec. Uri.encodeComponent already exists in dart:core, so the value here is byte-level control — encoding arbitrary bytes (not just valid UTF-8 strings) and choosing the unreserved set.

Architecture fit

• Standalone byte loop; bypasses the bit-regrouping engine entirely.
• Codec class with configurable "unreserved" set (RFC 3986 default); toPercent/fromPercent public API with codec: override.

Differential oracle: Uri.encodeComponent / Uri.decodeComponent for the UTF-8-safe subset.

Spec: RFC 3986 §2.1

Effort: S

Checklist

• Byte loop encode/decode, configurable unreserved set
• Six standard test groups incl. differential vs Uri.*
• Invalid-input group (truncated %, non-hex digits)
• README / CHANGELOG / example / test_integration updated
• Format / analyze / test (vm + node) green