Complete OverPy translations and custom-game settings support

[Teakowa]

Parent: #326

Goal

Complete the pinned OverPy translation and custom-game-settings source surface required for forward compilation.

Context

The current user-facing support contract still marks translation declarations / .po lifecycle and settings { ... } as partial. These are distinct user-visible OverPy capabilities but share project/preprocessing and final Workshop-output boundaries.

opy-rs owns OverPy translation/settings syntax, project state, source semantics, diagnostics, and OPY-specific lowering. Canonical Workshop settings schemas, localization identities, validation, and emission remain owned by workshop-rs.

Scope

• Complete pinned upstream translation declaration behavior and the supported #!translations project flow.
• Complete the upstream-defined translation call/output lifecycle needed by ordinary compiler usage, including .po interactions where they are part of the public compiler workflow.
• Complete the public OverPy custom-game-settings syntax instead of accepting only a schema slice.
• Support the public external settings-file form (settings "...", including the documented settings.opy.json workflow) in addition to inline settings { ... }.
• Cover main-file and included-file behavior, maps/heroes/modes, enum values, numeric ranges, extensions, and source diagnostics where upstream defines them.
• Preserve canonical validation and localized Workshop emission through workshop-rs.
• Route missing canonical settings/localization capability to workshop-rs and integrate it after the owner contract is available.
• Update the user-facing language support pages when behavior changes.

Non-goals

• Reimplementing the Workshop settings schema or localization catalog in opy-rs.
• Completing Workshop output-language locale coverage; that is tracked separately by Support the full pinned OverPy Workshop output locale set #333 and its canonical owner dependency wrightkit/workshop-rs#237.
• Reproducing editor-only UX that is not part of the compiler/project contract.
• Using Bastion-specific settings as the semantic specification.
• Treating a source file parsing successfully as proof that settings/localization semantics are complete.

Acceptance criteria

• Pinned upstream translation declarations and ordinary compiler translation workflows in scope are supported with equivalent source semantics.
• The public pinned OverPy inline and external-file settings source forms are accepted, validated, and lowered through canonical Workshop ownership.
• Invalid translation/settings forms fail at a meaningful source boundary with provenance-preserving diagnostics.
• Required Workshop schema/localization gaps are fixed in workshop-rs rather than duplicated locally.
• Independent upstream/differential or provenance-linked evidence demonstrates completed translation/settings behavior.
• Independent ablation of substantive implementation logic makes the corresponding evidence fail again.
• Translation and custom-game-settings rows no longer remain 🚧 Partial because of missing pinned core compiler behavior.

Ownership

• OverPy translation/settings syntax, project semantics, diagnostics, and OPY-specific lowering: opy-rs.
• Canonical Workshop settings schema, localization, validation, representation, and emission: workshop-rs.