Rewrite the buffer-based base64 placeholder as a faithful port of
ref/hare/encoding/base64/base64.ha over the just-landed io-streaming
surface (mirrors lib/encoding/hex).
Ships: std_encoding/url_encoding (module-level `def` consts; decmap
trailing 0xff run spelled out, no '...', to stay on #251 and avoid the
#250 repeat-fill sugar); the streaming encoder newencoder/encode/
encodeslice/encodestr with a padding closer wired into the inline
vtable; encodedsize/decodedsize; and decodestr as a direct in-memory
decode via decmap (the same divergence hex took for its direct path —
its return union carries errors.invalid, unconstrained by io.error).
Deferred (at-site notes): the streaming decoder newdecoder/decode_reader
(#247-sibling, blocked on #199b — io.error lacks errors.invalid).
clear() wipes the work buffers with explicit full-length slices
(`[0:len(...)]`) rather than Hare's bare-array decay (pending #258
[N]T->[]T coercion) to preserve the whole-array hygiene wipe.
base64 graduates off 900_stdlib (cross-module refs resolve only via
driver concatenation, as hex did); coverage at 984_base64_run over the
RFC 4648 §10 vectors for std and url.
Hojun-Cho
4d0d3b58e6