lib/encoding/hex: align to Hare io-streaming surface

The old buffer surface (encodedsize/decodedsize + encode(dst,src) i32 +
decode(dst,src) (i32|invalid)) does not exist in Hare — it predates the
#94 io vtable and mis-cited hex.ha:175 while implementing a different
signature. Replace it with Hare's real surface
(ref/hare/encoding/hex/hex.ha):

  - newencoder(out: io.handle) (:28) — write-only encoder stream.
  - encode(out: io.handle, in) (size | io.error) (:91).
  - encodestr(in) str (:68).
  - decodestr(s) ([]u8 | errors.invalid) (:175).

Divergences (documented at-site):

  - The streaming DECODER (newdecoder/decode_reader, :120,:129) is
    DEFERRED to #247, blocked on #199b: Hare's decode_reader returns
    errors::invalid, which fits Hare's io::error (spreads
    ...errors::error). ww's io.error (lib/io/types.ww:55-62) does not
    carry errors.invalid, and io.read's (size|eof|error) can't propagate
    it, so a hex decoder *stream* can't faithfully report invalid hex
    through io.read yet. decodestr ships as a direct transform meanwhile.
  - nomem dropped from encodestr/decodestr returns (ww memio.dynamic has
    no failure path — same memio.string rule-9 carve-out, memio.ww:208).
  - The local hex.invalid type is deleted in favor of errors.invalid
    (that was the original divergence).
  - encode uses a single io.write rather than Hare's io::writeall (ww has
    none — fmt.fprint:498-501: callers drive write-all over raw io.write;
    encode_writer is whole-slice so a single write is equivalent).
  - dump (:212) deferred: ww has no default-arg support and fmt's
    formattable lacks u64 (#209), so the address column can't be ported
    faithfully yet.

hex is now import-bearing, so it moves off the 900_stdlib standalone-
compile list (like fmt/os/strings/bufio/bytes/errors before it); coverage
stays at 979_hex_run.c. The stale "mirrors lib/encoding/hex.encode"
comments in lib/encoding/utf8/utf8.ww are updated, which regenerates the
6 selfhost combined.ww (5 cmd + test/smoke) (comment-only, byte-id-neutral).

selfhost/cmd/w6l/main.combined.ww

@@ -1551,8 +1551,7 @@ export fn rcut(in: []u8, delim: (u8 | []u8)) ([]u8, []u8) = {
 //
 //   - `encoderune` takes a caller-supplied `out: []u8` and returns
 //     the byte count. Hare returns a slice into a `static let buf`;
-//     the caller-buffer form mirrors lib/encoding/hex.encode and
-//     skips the static-buffer/slice-return pair.
+//     the caller-buffer form skips the static-buffer/slice-return pair.
 //
 // Deferred (no in-tree caller, follow-up tasks): `appendrune`,
 // `strencode`, `strdecode`. Hare's string-iteration surface
@@ -1849,8 +1848,8 @@ export fn utf8sz(c: u8) (i32 | invalid) = {
 // same precondition Hare asserts at ref/hare/encoding/utf8/encode.ha:9.
 //
 // Surface deviation: Hare returns `[]u8` (slice into a static buf).
-// ww uses the caller-buffer form (matches lib/encoding/hex.encode);
-// caller can reuse a [4]u8 stack scratch across encodes.
+// ww uses the caller-buffer form; caller can reuse a [4]u8 stack
+// scratch across encodes.
 export fn encoderune(out: []u8, r: rune) i32 = {
 	let ch: u32 = r: u32;
 	if (ch >= 0xD800u32) {