Group friendly fire: block at the source seams (3/4, part of #277)

[Zaldaryon]

Summary

Third of four stacked follow-ups to #300. Moves the group friendly fire block from the health check up to the two seams where an attack is actually decided, which fixes three of #300's stated limitations at once.

Base branch is feat/issue-277-ff-explosions (PR 2/4).

Why

The #300 hook in EntityPlayer.ShouldReceiveDamage stops the damage but runs late. By the time it fires, a melee swing has already:

• played sounds/player/slap (EntityAgent.OnInteract),
• spent a point of weapon durability (Collectible.OnAttackingWith calls DamageItem),
• fired OnPlayerInteractEntity, which combat-tags both players (StratumCombatLogSystem).

A blocked shot similarly still paid the arrow its durability and impact sound in ImpactOnEntity.

Change

• sources/VintagestoryApi/Server/StratumFriendlyFireHook.cs: adds OnBlockedAttack, an Action<string,string> the seams call so the notice logic lives in one place.
• sources/VintagestoryLib/Vintagestory.Server/StratumFriendlyFireGuard.cs (new): BlocksMeleeAttack, so the vanilla patch stays a one-line call.
• patches/VintagestoryLib/Vintagestory.Server/ServerSystemEntitySimulation.cs.patch: the guard call in HandleEntityInteraction, right after the vanilla PvP gate, only for attacks (flag), never right-click interactions like healing a group mate. Returning there is exactly the vanilla PvP-off path: no interact event, so no slap, no durability, no combat tag.
• patches/VSEssentials/Entities/EntityProjectileBase.cs.patch: the same rule inlined in CanDealDamage next to the existing AllowPvP check (VSEssentials cannot reference VintagestoryLib, so it uses the API-only types and fires the hook directly).
• sources/VintagestoryLib/Vintagestory.Server/StratumFriendlyFireSystem.cs: OnBlockedAttack points at a per-attacker throttled chat notice and a blocked-hit counter shown in /friendlyfire status.
• sources/VintagestoryLib/Vintagestory.Server/StratumFriendlyFireConfig.cs: NotifyBlockedAttacker (default true), NotifyThrottleMs (default 3000, clamped 500 to 60000), BlockedMessage. StratumConfig wires the EnsureSane call.
• docs/commands/friendlyfire.md: the new config keys and the status counter.

The ShouldReceiveDamage hook from #300 stays as the catch-all for mod-authored damage that calls ReceiveDamage normally, and as the test source for the explosion filter.

No combat tag on a blocked hit

This inverts what #300's body asked for. Today a blocked melee swing wrongly tags both players (the projectile path already does not tag, gated on didDamage). A hit that deals no damage is not combat, and letting a group mate tag you by holding left-click is an exploit. The melee seam returning before TriggerPlayerInteractEntity fixes it for free, no change to StratumCombatLogSystem.

Gates

• Two-pass build green, 4 pre-existing NU1904.
• bash scripts/smoke-test.sh: PASS ... 13 console command(s) verified.
• scripts/extract-patches.sh: ServerSystemEntitySimulation.cs.patch and EntityProjectileBase.cs.patch only. The SSE patch has one two-line brace reshuffle from the local decompiler in an unrelated adjacent hunk, otherwise the diff is the new melee hunk plus offset bumps. ServerMain.cs.patch from PR 2 is left untouched.
• Private Atlas suite: the control-path scenarios pass. The melee/projectile durability and no-tag scenarios need two connected players and are blocked by the same 1.22.7 synthetic-join gap as Stack multiple role prefixes on one player, ordered by Priority #274's suite.

Limitations

• With friendly fire off, swinging at a group mate no longer plays the slap sound. Intended.
• A mod that Harmony-replaces Entity.ReceiveDamage, the interaction handler, or the projectile path and skips the original still bypasses this. PR 4/4 logs a warning naming such a mod. No server platform can hard-prevent it.
• Blocked hits stay out of the combat log entirely (deliberate); the /friendlyfire status counter is the visibility.

Type

• Bug fix
• Performance
• New feature
• Refactor or cleanup
• Docs or build

Checklist

• scripts/extract-patches.sh ran clean.
• dotnet build VintageStory.slnx -c Release -p:EmbedPatchedFiles=true is green.
• Every vanilla edit has a // Stratum marker.
• No vanilla source committed.
• Tested on a real server start, not just compilation.

Related issues

Part of #277