Arena round 6: revert railgun_m damage, correct playtest record

Arena re-run after the playtest-1 adjustments:

- spam-cruiser arena landed at par (default +9%) - the range buff alone
  priced out the small-gun-spam meta, so revert railgun_m damage 16 -> 14
  (range stays 80). The buff had regressed drone-swarm-vs-cruisers to
  +47%, largely via a hit-count breakpoint (60 HP drone: 5 hits at
  14 dmg, 4 at 16).
- battleship +30% / dreadnought +29% vs pure railgun_s fleets accepted
  as reach-doctrine texture (the l gun's 130 m standoff working as
  intended); repair kept at 4 HP/s (below par in-fight is the correct
  price for free between-wave sustain).

Also corrects the playtest-1 record: it was two FULL playthroughs won in
~40 min each with cruisers only (not two pushes) - a 2.5-3x run-length
gap. Pacing knobs deliberately deferred to playtest 2, since those runs
predate the repair nerf and the l-gun siege range.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DyCu8vwChKMbLJQ3xosYEN

docs/balancing/derived.md

@@ -98,10 +98,12 @@ geometry-validated against the hull grids)
 see `history.md`)
 
 **Weapons:** railgun_s 2 dmg × 2.0 Hz (4.0 DPS), range 50 m;
-railgun_m 16 × 1.5 (24), range 80; railgun_l 52 × 0.8 (41.6), range 130.
-DPS per threat: s 0.62, m 0.55, l 0.41 — the concentration tax softened
-after playtest 1 so the ladder reads "pay some DPS efficiency for reach";
-railgun_l deliberately outranges stations (120 m) to buy the siege role.
+railgun_m 14 × 1.5 (21), range 80; railgun_l 52 × 0.8 (41.6), range 130.
+DPS per threat: s 0.62, m 0.48, l 0.41 — the concentration tax stands;
+reach is the bigger guns' compensation (ranges raised after playtest 1,
+which alone priced out the small-gun-spam meta). railgun_l deliberately
+outranges stations (120 m) to buy the siege role. railgun_m damage is
+breakpoint-sensitive: 15+ drops a 60 HP drone from 5 hits to 4.
 
 **Hull HP** (15/threat prior + empirical trims): drone 60, frigate 300,
 destroyer 550, cruiser 1500, battlecruiser 2400, battleship 6300,