Add scripted lightning weapon mods with interpolated anchored effects

mods/lightning_cannon/README.md

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+# Lightning Cannon
+
+An independent, reusable heavy lightning special for **825,000 credits**.
+Enable **Lightning Cannon** in the Mods menu, apply selections at the main menu,
+then buy it in the supply room. Select it in Inventory or cycle specials;
+**Fire or Special** discharges the cannon while selected. It consumes no ammo,
+and you can own one.
+
+The bolt instantly hits the nearest visible air or ground target ahead of the
+muzzle, within 180 logical pixels and a 36-pixel horizontal aiming allowance.
+It deals **32 damage**, then jumps from the struck ship to the nearest unhit
+enemy within **48 pixels**, for **20, 12 and 8 damage**. A discharge hits at most
+four different enemies. Chaining can change direction and continues from a
+destroyed target's last position. An isolated target takes only the initial hit;
+off-axis targets require an adjacent chain target. Cooldown is **18 ticks**,
+about 0.77 seconds. A missed shot shows a bolt and firing sound without damage.
+
+The 30-second controlled simulation measured **41.60 damage/s** on an isolated
+aligned target and **93.60 combined damage/s** across a four-target chain. The
+price places it above the original Pulse Cannon and below the Death Ray. See
+the package [balance report](balance-report.json) for the recorded observations.
+
+This package uses the public mod API's `scripted` weapon behavior, read-only
+entity snapshots, `damage`, `arc` and `play_sound` commands. All acquisition,
+chaining and damage rules live in `scripts/weapon.rhai`. It has no dependencies
+or overrides. It coexists with the other independent weapons and Iron Coast.
+
+The equipment icon ships in **16×16 SD** and **64×64 HD**, alongside the
+transparent source master and deterministic synthesized audio. Connecting bolts
+use the game's arc renderer at both scales. See [provenance](ART_PROVENANCE.md)
+for the exact built-in imagegen prompt, asset hashes and reproduction recipes.
+
+From `RustRaptor/`:
+
+```sh
+cargo run --locked --no-default-features -- --mods mods --validate-mods lightning_cannon
+cargo test --locked --no-default-features --test weapons_mods lightning_cannon
+```
+
+See [weapon pricing and measurements](../../docs/weapon-balance.md),
+[the API](../../docs/modding-api.md) and
+[the AI mod checklist](../../docs/mod-authoring-checklist.md).