Select LocalMath CI conservatively and document owner validation

[PraneethMerugu]

Select LocalMath CI conservatively and document owner validation

R04 of the ecosystem delivery map. This PR contains only workflow selection and
contributor documentation; it is separate from the structured-value companion.

Changes

• Keep the complete package suite and strict documentation build on every PR.
• Select scientific and actual-Metal checks from the entire PR diff. Only an
  explicit set of non-executable prose/metadata paths can omit those checks;
  unknown, executable, dependency, and workflow changes retain them.
• Fail clearly if change selection fails or returns an invalid value.
• Avoid duplicating the macOS smoke with the main/manual full macOS job.
• Correct contributor documentation to describe LocalMath's own examples,
  actual Metal test entry point, and separate model/visualization ownership.

Validation

• Historical pre-recovery complete owning package run: 1,500 assertions passed;
  the strict documentation build also passed then. These are not claimed as
  fresh recovered-branch CPU/documentation results.
• Recovered branch's full declared actual-Metal suite: 207 assertions passed.
• Independent review of the two-file diff and current base completed; no
  execution implementation, test oracle, dependency, or runtime API changed.
• Fresh hosted run34294535400 passed the complete package, strict docs,
  scientific tests, macOS smoke and actual Metal jobs. The main/manual-only
  full macOS job was skipped, not tested. Independent review is complete.

Base main was reverified at b699002 before publication; head is 135afe4.
This CI change does not add a new scientific, replay, or backend guarantee.