Ram: finisher-only by default, and the bullet-rain abort now measures real energy

Follows the diagnosis that proactive straight-line ramming CANNOT work: both bots
have MAX_SPEED=8, so a pursuit cannot catch an evading equal-speed opponent.
Measured over 49 rounds per arm, opportunity -> contact was **0/6** (base), 0/40
(ring), 0/12 (ringhot). The only proactive conversion in the whole corpus came
from a FINISHER, and only because a <20-energy DrussGT stops fleeing (that episode
closed at 6-8 px/tick). Opportunity episodes never got below ~80px; one ran the
full 60-tick duration cap and closed only 198->171px; a perfectly aligned
full-speed one closed 195->114px then plateaued.

CHANGES
- **Finisher-only default.** `finisher` (<20 energy, dist<300, we are healthier)
  and the rare `desperation` (both <5, dist<150) are kept; `opportunity` and the
  speculative `plan` are OFF. Both are env-reenableable with no rebuild:
  `TR_RAM_OPPORTUNITY=1` (tune via TR_RAM_OPP_DIST/MARGIN) and `TR_RAM_PLAN=1`.
  Justification: it removes 100+ non-converting episodes per fixture at zero
  measured loss (oldram vs base was p=0.69, damage 279 vs 284, survival 17/49 vs
  16/49) - and each of those episodes spent up to 60 ticks driving STRAIGHT at
  the enemy, abandoning the mover's dodging and disrupting aim.
- **`desperation` KEPT** deliberately: it is cheap and rare, fires only when both
  bots are nearly dead at short range (a coin-flip where 0.6 contact can decide
  it), and it is not the refuted straight-line pursuit.
- **THE BULLET-RAIN ABORT WAS DEAD CODE AND IS NOW FIXED.** `onHitByBullet`
  accumulated raw bullet FIREPOWER while `TR_RAM_ABORT_DMG = 0.5` was documented
  as a DAMAGE rate - so the bar was implicitly "sum of power > 7.5 over 15 turns"
  and the maximum rate ever observed was 0.27. It now accumulates REAL ENERGY via
  a `bulletDamage(power)` helper matching the server's `4p` / `6p-2` formula, and
  `TR_RAM_ABORT_DMG` defaults to **2.0 energy/turn** (~30 HP over 15 turns):
  "abort an in-progress ram if we take > 2.0 energy per turn". Same effective bar
  for normal firepower, and it can now actually fire - the live run reports
  `dmgRate=1.07/turn` where the old units said 0.27.
- **`ramStuckTicks` REMOVED.** It required `dist < 5px`; contact occurs at ~36px
  (two 18px radii) and position rewind prevents getting closer, so it could never
  increment. Only the 60-tick duration cap can now self-end a ram.

LIVENESS (measured, default config, vs a charging Java RamFire, 3 rounds):
  default              -> `[ram] ON reason=finisher` x3, `reason=opportunity` x0
  TR_RAM_OPPORTUNITY=1 -> `reason=opportunity` x4, `reason=finisher` x2
So the opportunity states DID occur and are suppressed by the new default - the
removal is real, not an arm that never fires. A line also read
`[ram] OFF reason=duration dmgRate=1.07/turn`, confirming the new energy units.

Adds docs/ramming_negative_result.md (70 lines) recording the question, the five
diagnostic answers, the geometric reason, the finisher exception, the two dead
code paths, and an explicit "do not re-attempt a proactive straight-line ram; if
point-blank forcing is ever wanted it is an INTERCEPTION/cornering movement
problem" note - the same pattern that stopped the corpse bug recurring.

Guards: test_ram_decision 40 (was 28), test_gun_harness 39, test_vbullet_metric 11,
test_power_selection 3, test_adaptive_radar 41, test_tfil_ring_weights 24,
test_power_policy 26, test_rack_membership 48, test_selector_tiebreak 19,
test_tm_pattern_registration 20, test_vbullet_admit_gate 12,
acceptance_offline_vs_online 12/12. ModularBot compiles.

Honest note: the abort-threshold fix is a real (tiny) behaviour change, NOT
measured-neutral - it only bites while a finisher ram is under sustained fire,
which is exactly the user's stated wish. The finisher-only removal itself is
measured-neutral per the given A/B.

docs/feature_ab_results.md

@@ -25,6 +25,9 @@ p=0.69, damage 279 vs 284, survival 17/49 vs 16/49 (p=1.0), ram contacts 1 vs 2.
 provably LIVE (6 `opportunity` ON events vs **0** under the old 50px/+30 gates, and `base` reached
 <40px on 10 ticks vs 0) — but it converts to essentially no extra collisions and no measurable
 outcome change. Safe to keep; it is not earning its keep, and reverting it is equally defensible.
+**Follow-up:** a log-replay diagnosis later showed `opportunity -> contact` was **0/59** and the
+gate never got below ~80px, so the default is now **finisher-only** and the proactive trigger is
+opt-in (`TR_RAM_OPPORTUNITY=1`). See `docs/ramming_negative_result.md`.
 
 **3. Range-weighted ring mover — HURTS the objective. Do NOT ship.** See below; this is the
 important one.