feat(cybergym): v3 lane bottom-fill payout curve (leader takes remainder)

[ai-hpc]

Implements jared's v3 CyberGym-lane payout curve (STAGE A of the v3 lane redesign). With fewer than five qualified miners, the bottom ranks take their fixed level shares from the bottom up and rank 1 absorbs the remainder (winner-take-most), replacing the old renormalize:

N	shares
1	[1.00]
2	[0.96, 0.04]
3	[0.89, 0.07, 0.04]
4	[0.79, 0.10, 0.07, 0.04]
5	[0.65, 0.14, 0.10, 0.07, 0.04]

Level scores are the existing TOURNAMENT_SHARES; shares always sum to 1. The change is byte-identical in cathedral-validator and cathedral-distill cybergym_tournament.py (BOUNDARY.md fork hazard), guarded by an identical golden test in both.

N=0 (empty lane) is unchanged here; the N=0->compute redirect is a follow-up (it touches the validator re-derivation and is a v3 contract-shape change).

Reward-path; not self-merged.

🤖 Generated with Claude Code

[ai-hpc]

Co-merge required (fork hazard). This _award_shares change is duplicated byte-identically in the sibling repo (cathedral-validator #146 ↔ cathedral-distill #161, per BOUNDARY.md — the two vendored cybergym_tournament.py copies must stay identical). An adversarial review confirmed they are byte-identical today and that the golden test test_award_shares_curve_is_the_fixed_v3_schedule pins the same table in both — but nothing asserts cross-repo identity, so these two PRs must land together; merging one without the other silently diverges the live payout curve from the vendored copy. (The idle-redirect PR #147 is independent — different files — but also assumes this curve on the live path.)