Expose opt-in classic combat physics and off-hand eligibility APIs #14376

Is your feature request related to a problem?

Plugins supporting classic combat on modern clients need native hooks for projectile collision/flight, player knockback and movement, and off-hand equipment eligibility. Replacing native simulation or repeatedly correcting velocity/inventory after events can lose the original collision state or produce prediction mismatches. Modern vanilla behavior is intentional; this is an optional extension request, not a vanilla bug report.

Describe the solution you would like

Expose default-native, opt-in runtime capabilities:

• Player classic combat physics: legacy component clipping/jump arithmetic, accepted knockback, ordinary fall/fire behavior and classic explosion damage/attribution, with modern weapon impulses retained.
• Arrow, throwable and fishing-hook legacy flight/intersections/owner grace; initial launch position through a generic projectile API. Launch velocity and gameplay policy stay with the plugin.
• Player off-hand equipment eligibility, including native menu routing, merge candidates and indirect held-item selectors. Stored residual items are retained. Plugins still control swap/creative/interact/resurrection events through existing public APIs.

Native damage acceptance, event cancellation, protection checks and ownership remain authoritative. No global combat configuration, balance preset, custom item identity or anti-cheat exemption is included. Matching client and anti-cheat prediction remains the integrator's responsibility.

Describe alternatives you have considered

Existing combat plugins already handle configuration, damage/event policy, cooldowns and launch values. They cannot select the native flight/intersection order or all equipment consumers through current public APIs. Packet simulation or copying native engines increases maintenance and risks bypassing native event/protection handling.

Other

The proposed implementation is isolated on the maintained ver/26.2 branch, current base 58e6e45d67c998909581f9079189ef51eb6f1267. It has been validated there, not against main/26.3. Default-native regression controls are included. API/server checks and Paperclip assembly passed: 519 API and 9330 server XML executions, no failures/errors, 2/22 skips (suite duplication means these are not unique test counts). No connected-client campaign is attributed to this public export.