refactor(api)!: drop the inert config and charset surface

[andiwand]

🤖 Generated with Claude Code

PR 3 of the v7 API plan.
Same premise as #832 — surface that was kept only so existing callers kept
compiling, removed now that a major allows it.

HtmlConfig

Four fields stored a value no view ever read back:

field	dead since
background_image_format, background_image_dpi	no view renders a background image to a file
no_drm	no output carries a restriction to lift
embed_outline	an outline is never written

Gone with their java, python, objc and wasm mirrors — including the
@deprecated entries in index.d.ts, so a TypeScript caller finds out at
compile time.

TextFile::charset() and UnknownCharset

charset() was text_encoding_to_string(encoding()) with a nullopt for
unknown. encoding() says the same thing as a TextEncoding and reports
TextEncoding::unknown instead of an empty optional. UnknownCharset goes
too — nothing throws it, because a file whose encoding cannot be named is still
text.

The bindings keep their own charset(). Worth being explicit, because it
looks like an oversight and is not: those accessors are not mirrors of the
removed overload — JNI, python and objc each build the string from
encoding() directly. Removing them would lose the accessor rather than
redirect it, because TextEncoding is not mirrored in any binding. So a
binding-side removal has a prerequisite, which turns out to matter beyond this
PR:

Mirroring TextEncoding (the enum, plus text_encoding_to_string,
text_encoding_by_name, text_encoding_names, text_encoding_is_decodable,
all_text_encodings) is a non-breaking addition, and it is also what PR 7
needs before CsvOptions can be folded into the decode options — CsvOptions
carries a TextEncoding. It becomes its own PR, before 7.

That reordering is the one thing here that was not in the plan when #831 was
written; I will update the plan doc on that branch.

Verified

Library, CLI, JNI + jar, python module and odr_test build clean; 69 python
tests and the 57 *Csv*/*Text*/*Encoding* gtests pass.

Migration

• config.background_image_*, config.no_drm, config.embed_outline — delete.
• text_file.charset() → text_file.encoding(), comparing against
  TextEncoding::unknown instead of checking the optional. text_encoding_to_string
  gives the old string back if you want it.