Single-language restriction profiles (e.g. JavaScript-to-JavaScript mode)

[konard]

Context

Issue #47: "if we want to keep working with single language for example for
javascript to javascript transformation - we just restrict ourselves with
using only features of meta language that JavaScript supports exactly". No
symbol in src/ models per-language capability profiles today. See
requirements.md R-16 and
solution-plans.md S-11.

Blocked by: #10 (a profile is effectively the domain of a translation
rule set).

Scope

• LanguageProfile links: per-language capability sets naming the concepts,
  link types, and translation rules a target language supports - profiles are
  links, hence queryable and user-editable like everything else.
• A profile can be declared by the user or computed from a #10 rule set's
  domain.
• ParseConfiguration::with_profile(...) (or a transform-time argument):
  operations that would leave the profile fail with a diagnostic link naming
  the unsupported feature instead of producing untranslatable output.
• Ship a JavaScript profile as the reference case with a JS→JS transform test
  that stays within profile and one that is correctly rejected.

Acceptance criteria

• Profiles are representable as links and queryable.
• In-profile JS→JS transform passes; out-of-profile operation yields the
  documented diagnostic.
• Profiles compose with #10 rule sets (declared or computed domain).
• Changelog fragment added (bump: minor).

References

• Requirement: requirements.md R-16
• Solution: solution-plans.md S-11
• Part of We need to carefully compare all competitors in all scopes with out project, and make sure we have the richest features set #47; work lands on branch issue-47-76af108c0f24 (PR Finish issue #47 parity feature set #48).

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Filed from docs/case-studies/issue-47/proposed-issues/11-language-restriction-profiles.md. Part of the implementation plan for #47.