plugin: index leaves incidental languages out of a build that names no --lang (#1835)

[swapnilpaliwal-sd]

Closes #1972

Part of #1835.

What

With no --lang, axiomcode index built a graph for every language it found and chose the main language by file count. One .py file beside three trivial .js files became a JavaScript repository with a Python graph beside it. A Python backend with a few .js build scripts got a second graph, a second section in every answer, and a cold JavaScript engine compile.

The rule (ax_fresh.py choose, read by axiomcode-build)

• Main language: the one with the most code. Code is counted in non-blank lines, with each file capped at 1,000 lines so a vendored file cannot decide it.
• Incidental language: any other language with fewer than 20 source files AND either under 5% of the code or under 100 lines in all. It is not built.
• Named, not hidden: every index ends by naming what it left out and the --lang that includes it, for example:

  not indexed, incidental to this python repository (under 20 files, and under 5% of its code or 100 lines): javascript (3 files, 3 lines); to include: axiomcode index --lang python,javascript
  

• An explicit --lang always wins. Web (HTML/CSS) follows the same rule.
• Each language is still its own graph. Nothing is linked across languages.

How 20 files and 5% were chosen

• Measured on 42 corpus repositories, 14 each of Python, Java and C#. 35 have a second language, 79 second languages in all.
• 41 of those have under 20 files, and they split with a gap: 33 are at 3.8% of the code or less (stray scripts, a docs theme, samples), and 8 are at 6.0% or more (small frontends, a site's pages). 5% sits in the gap.
• Every second language with 20 or more files is still built. 26 of the 42 repositories still get more than one graph.
• Weighting by code changes the main language in 2 of the 42: two Python projects that file count had called TypeScript and JavaScript.

Consistency

Every reader of the main language (queries, context, the hooks, MCP) takes it from the built graph's run.language, so they agree with the build without changes of their own.

Docs

README Get Started shows axiomcode index --lang <language> and states the rule in one sentence.

Tests

• tests/incidental_languages.py, 18 checks, all pass:
  • 1 py + 3 js gives python only, with the note.
  • 40 py + 3 js build scripts gives python only, and answers have one section.
  • Control: a balanced TypeScript + Python repository builds both graphs.
  • Control: --lang javascript gives javascript only.
  • --lang python,javascript builds both.
  • On the base, the first two shapes build two graphs, and both controls hold.
• tests/run.py --lang python: 445/445. tests/engine_choice.py passes.
• tests/multi_language.py: 4 failures, all of which also fail on the base (6 failures there).
• tests/publish_order.py: the same 5 failures as on the base, by name.
• Fixture changes, and why:
  • The multi_language.py and publish_order.py fixtures had 5-16 lines per language, which makes them incidental under the new rule. Each gets a filler module of 100+ lines per language that no check reads.
  • The maven-web check now expects its single 6-line Java class to be named as incidental instead of built.