Make hunter the baseline FFA default (legacy becomes the explicit opt-out)

[devin-ai-integration]

Summary

Reopens #51 (closed unmerged; same commit, rebased onto current main at 85159b0). Flips the compiled default of the baseline policy's FFA doctrine from legacy to hunter — the arm #46 landed behind an env flag now ships on:

-  FfaDoctrine = FfaLegacy
+  FfaDoctrine = FfaHunter
   ...
-    if requestedDoctrine.len == 0: FfaLegacy
+    if requestedDoctrine.len == 0: FfaHunter

CTF_BOT_FFA_DOCTRINE=legacy is now the opt-out; every other arm (hybrid, passive, rush, shade, hunter) still selects by name, and no hunter tuning constant, other doctrine, or sim file changes. tools/ci/check_gameversion.sh origin/main is quiet — GV45 unchanged, no rule change claimed, so replays and fixtures are untouched.

Why hunter: it arms itself (bounded, abortable gun trips), fires at real weapon range instead of a flat 120px, pursues only weaker targets while healthy, and starts the endgame at 4 alive. Locally that is +42.3 ± 7.8 score over passive with survival up (89s vs 64s) and armed fraction 0.07 → 0.51 (#46). The honest caveat, also from #46: on the hosted field against the current champion it measured +3.8 ± 3.4 pp pairwise across 196 clean episodes — inside noise. So this is defensible as "the bundled bot should fight rather than hide", not as a proven ladder win.

What it actually changes in production: the bundled baseline player artifact in coworld_manifest_battleroyale.json is built from this source and is what certification, smoke runs and league seat-filling use, and the manifest carries no doctrine override — so the flip reaches the league only on the next Coworld publish, and competitor-submitted policies are unaffected.

Tests move with the default rather than being relaxed: test_baseline_doctrine.nim now asserts hunter-when-unset and adds a case asserting legacy still resolves explicitly. tools/run_ffa_demo.sh's unset fallback follows the same default so the demo matches shipped behavior. docs/ENV_VARIATION.md and players/baseline/README.md are corrected — both previously stated the league baseline stays legacy unless a runner opts in.

Verification

Carried over from #51 on the identical tree: nim c -d:release players/baseline/baseline.nim; startup line for unset (ffaDoctrine=hunter) and each explicit arm, plus invalid-doctrine exit 1; tests/test_baseline_doctrine.nim (4 tests) and tests/test_artlog.nim; tools/ci/check_gameversion.sh origin/main (GV45 unchanged both sides). Runtime: two real 12-player FFA matches from one identical config with bots launched directly, so the compiled default is what was under test — unset gave 12/12 ffaDoctrine=hunter banners, 21 loot trips, hunter-only engage reasons (fire_range 192, pursue_weak 31) and an elimination ending; explicit legacy gave 0 loot trips and 0 hunter engage reasons.

Link to Devin session: https://app.devin.ai/sessions/274a6b4a46034710a7a81a2a81dc02dc
Requested by: @aaln

[devin-ai-integration]

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