codegen: text-format printer (pb.text.encode + <Msg>_text wrappers)

Adds a descriptor-driven text-format encoder mirroring `protoc --decode`
output: one field per line, 2-space indent, octal byte escapes,
`nan`/`inf` floats, `opts.single_line=true` for compact one-liners.
WKT-aware — Timestamp/Duration accept datetime cdata or {seconds,nanos},
wrappers print their unwrapped scalar, Struct/Value/ListValue walk the
tagged-table form, FieldMask flattens to `paths:` lines, Any stays
opaque.

Both codegen modes emit `M.<Type>_text(t, opts)`, surfaced as `pb.text`
on the public table. Encode-only; matching parser deferred.

Tests cover scalars, repeated, maps, oneof, optional presence, all WKT
types, single-line mode, and the generated wrapper across both modes
(74 cases). 403/403 luatest green, 19/19 jit-trace gate green.