docs: add a fork guide for target languages, prompts and backends

[0xra0]

What

A new FORKING.md for people who fork this editor to add their own target language, their own translation voice, or their own quality checks — and which of those need a fork at all.

Why

Adding a translation target language touches about eight tables, and every one of them is a dict.get() with a fallback. A fork that registers the code and stops gets a working picker and silently worse output, with nothing logged:

>>> EncodingConverter.get_encodings_for_locale("cs")
('utf-8', 'windows-1252')      # wrong — Czech is Central European
>>> EncodingConverter.get_encodings_for_locale("czech")
('utf-8', 'windows-1250')      # the entry that exists
>>> default_style_rule("cs")
'Write natural, polished cs appropriate to…'   # the bare code as the name

Czech is the worked example precisely because the repo is already half-wired for it (spell_checker.LANG_TO_DICT and quality_checker._LATIN_SCRIPT_TARGETS know cs; a cs_CZ UI translation ships), so it shows both what you add and what quietly defaults to something wrong.

Contents

1. What doesn't need a fork — Prompt Editor, glossary, protected terms, player gender, Modelfiles, themes, BSE_CONFIG_DIR
2. Adding a target language — the one-line minimum, then _LANG_DISPLAY, _TARGET_STYLE, ENCODING_PAIRS, _GENDERED_TARGETS, the four-file word-list procedure, spell check, QC wiring, optional language-specific checkers, font/width verification, and a 10-row checklist
3. Custom prompts — the three layers with no code, then the full assembly order of to_system_prompt() and how to add a PROMPT_DIALS entry
4. Adding a backend — the worker signal interface and where routing happens
5. Other extension points — themes, _FIELD_DEFS, string categories, lore RAG, new settings
6. Keeping a fork mergeable — every step in §2 is an entry in an existing table, never a modified line

Two findings worth flagging on their own:

• Prompt rule 3(a) ships ~30 Ukrainian bracket-token examples ([Lie]→[Збрехати], …) to every target — ask for German and the model still sees a wall of Cyrillic. Documented so a fork knows to swap them.
• The Prompt Editor's language combo is built from _LANG_DISPLAY, not SUPPORTED_LANGUAGES, so a language added only to the latter cannot be given a custom Rule 1 through the UI.

Interface language vs translation target

These were easy to confuse, so both files now open by naming the distinction, and TRANSLATING.md's ambiguous "Adding a new language" heading is now "Adding a new interface language" (its in-page anchor updated to match).

Verification

Every factual claim was checked against the live modules with a 32-assertion script rather than written from memory — symbol names, fallback values, which sets gate which check, and the get_prompt_customizations() == ({}, "", {}) contract. docs/forking.rst follows the contributing.rst convention (summarise and link out rather than keep a second copy that drifts); Sphinx builds clean under -W.

No Python changed.

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