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feat(encoding): add encoding/percent for RFC 3986 URI components (#4214)

Add `encoding/percent`, a string-to-string percent-encoding codec for URI
components, so applications stop hand-rolling it or shimming JavaScript's
`encodeURIComponent`/`decodeURIComponent` (moonbitlang/openseek#1408).

- `encode` converts the text to UTF-8 and escapes every byte outside the
  RFC 3986 unreserved set as uppercase `%XX`. It walks UTF-16 code units
  itself and replaces unpaired surrogates with U+FFFD, because
  `@utf8.encode` panics on them on the non-JS backends.
- `decode` decodes every `%XX` (either hex case) once, interprets each
  maximal escape run as UTF-8, copies literal code units through unchanged
  (including `+`, BOMs and unpaired surrogates) and raises `Malformed` with
  the whole input on a bad escape or invalid escaped UTF-8.
- `decode_lossy` keeps a malformed `%` literally, resumes at the next code
  unit, and replaces invalid escaped UTF-8 with U+FFFD; it agrees with
  `decode` whenever `decode` succeeds.

Literal spans are sliced with `StringView::view`, which checks bounds but
not surrogate boundaries, so views that split a pair are handled instead of
aborting. Tests cover the ASCII table, UTF-8 boundaries, surrogate cases,
nonzero-offset views, the malformed table from openseek, lossy replacement
counts, and quickcheck properties against an independent UTF-16 repair
oracle. Ten single-point mutations each fail between 2 and 8 of the 24
tests.

Codex CLI review: "No blocking issues found ... An independent
transcription check passed for every code point and surrogate pair."



Claude-Session: https://claude.ai/code/session_01MVKQs2BgHmc3VBDXBU8AgD

Signed-off-by: Codex CLI <codex@openai.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

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