Translate howto/free-threading-python.po - #1266

Translate howto/free-threading-python.po (Python support for free threading).

• Translates all 58 untranslated entries and fixes the PO header (Project-Id-Version: Python 3.15, translator, revision date).
• Terminology follows glossary.po and existing usage: 自由執行緒, 全域直譯器鎖, 擴充模組, 不滅, 參照計數, 建置 for "build".
• sphinx-lint passes. I rendered the page against the 3.15 CPython docs: all 58 entries show up in the built page and no Chinese touches Latin text without a space.

Points for reviewers to decide:

• The two flag descriptions keep true / false in English (預設為 true / 預設為 false) instead of 真 / 假, because 預設為假 reads unnaturally. The rest of the repo uses 真 / 假, so I'm happy to switch back.
• "per-thread reference counting" is 執行緒各自的參照計數 (with the English term at first use), because 每執行緒 reads awkwardly.
• "thread-local objects" is 執行緒局部物件, matching c-api/tls.po and whatsnew/3.13.po. library/decimal.po and whatsnew/3.12.po use 執行緒區域.
• Terms with no existing precedent in the repo: 駐留 (intern), 配置器 (allocator), 堆積 (heap), 位元組碼求值器 (bytecode evaluator).

Also includes a small docs commit for the translate-po skill (.claude/skills/translate-po/), found while reviewing the rendered page:

• \\ renders as nothing, so it removes the required CJK-Latin space. Use a plain space before roles that render Latin text (:ref: without a title, :mod:, :func:, :class:), and keep \\ for roles that render Chinese (e.g. :term: with a translated title). The skill's examples taught the wrong default and are fixed.
• Added the README's parentheses rule: English-only brackets are half-width with spaces around them, Chinese content uses full-width; annotate a term on first use only.

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