[Roxas2712]

Adds set_field(key, value?) and set_option(key, value?) to API-3 portable scripts. Private mod:key fields use only the owning mod's save bucket; options retain native literal-key semantics. Omitted values delete the selected entry.

Depends on #119 and its prerequisites; merge that first. Focused review: https://github.com/Roxas2712/Gen2Recomped/compare/8a6eca8ebcc5501bc390f39de6aeba75b2878d48...5528ea28a9e8f9451aa27bbc6f2d411fa8ef2e62

Values are bounded acyclic plain data, snapshotted on compilation and copied on every execution. Store shapes are checked before admission and again at dispatch. A linear preflight accounts for an earlier options replacement. Native root-field and option semantics remain game-specific; runtime store changes can still stop later rows. Replacing the entire root modData store is deliberately refused, unlike the unrestricted reference write, so private-field ownership cannot be bypassed.

Validation:

• 72 targeted assertions: all eight versions, mutation isolation, deletion/false values, invalid/changed stores, foreign namespaces, executable/nonfinite/cyclic/metatable data and aggregate limits.
• 168 cross-engine comparisons / 336 isolated runs at clean feature and combined integration heads.
• Feature 94/104 engine suites; integration 99/109. Exact baseline 17 failed assertions and 10 failed suites unchanged; SDK 3/3 and B30 pass.
• Formatting and diff checks pass. No imported-ROM gameplay claim; writes do not automatically save to disk.

Hosted workflows await maintainer approval for this fork (action_required): API 3, ci. No hosted pass is claimed.