Add snapshots patching plugin API

[ObelardO]

Summary

• Add minimal hook calls in SV_BuildClientSnapshot so plugins can modify outgoing per-client snapshots without rewriting snapshot build logic.
• Implement snapshot patch dispatch in new plugin_snapshot_patching.c (registration, chained multi-plugin callbacks, sorted append for injected entities/clientStates).
• Expose plugin API: three patch hooks (playerState, entity, clientState) with SNAPSHOT_PATCH_MODIFY / SNAPSHOT_PATCH_APPEND modes, plus archived client lookup helpers for killcam replay.

Design

• sv_snapshot.c only calls thin wrappers; all plugin-specific logic lives in plugin_snapshot_patching.c.
• Multiple plugins can register hooks; callbacks run in plugin slot order (chained MODIFY, each plugin may APPEND).
• function_declarations.h and plugin_exports.c export register/unregister and Plugin_SV_GetArchived* helpers.

Test plan

• Server builds on Windows/Linux
• Plugin loads and registers snapshot patch callbacks without errors
• Existing plugins unaffected when no snapshot patch is registered
• Two plugins registering snapshot hooks both receive MODIFY/APPEND calls in order
• Archived client origin/entity/state helpers return expected replay data during killcam

Notes

Companion plugin (FinalKillcamEntityCamera) lives on fork branch final-killcam-plugin and is intentionally not included in this PR.

[proxict]

Hi, could you please elaborate what your goal is with these changes?

[ObelardO]

Hi! I'm currently working on a couple of plugins for my server:

1. An anti-cheat plugin that can modify outgoing snapshots. My goal is to strip redundant data, for instance, based on player distance.

2. A final killcam plugin capable of replaying match events with specific modifications. For example, a third-person killcam for entities like explosives, vehicles, or killstreaks. The stock game has an issue where the killcam's camera angle is calculated client-side and is bound to the player watching the killcam rather than the player getting killed in it. To avoid modifying the client files, I'm using some entity-spoofing workarounds.