Refactor duplicated site scripts into shared JS/gf-utils.js helpers

[devin-ai-integration]

Summary

The documentation site repeated the same few patterns dozens of times (13 copy-pasted partN() loaders in app.js, 40 identical _keyCustom blocks in code/part-eight.js, 4 identical ajax callbacks in code/part-two.js, ...). This adds JS/gf-utils.js — a small layer on top of GF-1.js, loaded by index.html right after it — and rewrites those call sites in terms of it. Net −335 lines, no behaviour change.

New helpers: _bindClick(idsOrMap, fu), _bindKey(codeMap), _printKeyTo(id, labelMap), _responseTo(id), _alertUrl(url), _nextIndex(i, len), _printAll(idContentMap).

app.js: the 13 partN() functions, their 14 _onClick bindings and the 14-branch if/else chain behind INSERT collapse into one table + one loader:

const documentationPart = [{ file: "part-one", trigger: ["part_one","part_one_one"] }, /* ... */];

function loadPart(i) {                     // i indexes documentationPage[1..13]
  const part = documentationPart[i - 1];
  _loadDoc("./code/"+part.file+".html", function(res){ /* same body as before */ });
}

documentationPart.forEach(function(part,index){ _bindClick(part.trigger, function(){ loadPart(index+1); }); });

INSERT keeps the old semantics (home → part1, partN → partN+1, part13 → part1) via documentationPage.indexOf(now_page), and still does nothing when the current page is Get-Start/All-Function.

One intentional behaviour fix: the PREVIOUS key in code/part-eight.js used to _writeLog("PREVIOUS") and log the wrong key code instead of printing to display_key — the table-driven version makes it behave like every other key.

Verified in Chrome against a local static server: parts 4, 8, 9 and 11 render, nav clicks and INSERT navigation work, key labels print, and the color cycling in part 11 still rotates.

Link to Devin session: https://app.devin.ai/sessions/fc017cd138aa4772a02a07db257dbf5d
Requested by: @lamhotsimamora

[devin-ai-integration]

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