docs: define encoding representation contract

[vycdev2]

Summary

• define byte-native strict Base64 and hex codecs with fallible decoders
• separate RFC 3986 URL component semantics from form-component +/space behavior
• record deterministic errors, trusted stdlib/runtime boundaries, backend obligations, and staged regression coverage
• align the design, architecture, progress tracker, and docs index

Verification

• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed (workspace suite, including all 432 driver fixtures)
• git diff --check — passed

Risk

• Low: documentation-only proposed contract; implementation remains pending and the PR makes the migration explicit.

Closes #71

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.

[vycdev]

The encoding contract reviewed as technically sound, but this documentation branch now conflicts with current main after the earlier architecture/design/progress updates landed. Please rebase and reconcile the current canonical text rather than overwriting it, then rerun diff/link checks.