feat: collapse emission split constants

[CaptainTimon]

Summary

• Collapse the top-level emission constants to OSS_EMISSION_SHARE = 0.90 and ISSUES_TREASURY_EMISSION_SHARE = 0.10
• Remove the fixed ISSUE_DISCOVERY_EMISSION_SHARE and RECYCLE_EMISSION_SHARE constants from the active blending path
• Blend the current OSS and issue-discovery reward vectors into one normalized scoring pool that receives the 90% OSS allocation
• Recycle only unclaimed scoring-pool slack when no OSS or issue scorer is active
• Add tests for the collapsed constants, active scoring allocation, and empty-pool recycle behavior

Related Issues

Part of #1215. Covers required deliverable 9 only.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

uv run pytest tests/validator/test_forward_emissions.py -q
uv run ruff check gittensor/constants.py gittensor/validator/forward.py tests/validator/test_forward_emissions.py
uv run ruff format --check gittensor/constants.py gittensor/validator/forward.py tests/validator/test_forward_emissions.py
uv run pytest -q

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

Note for reviewers: this PR intentionally only collapses the top-level constants and removes the fixed recycle baseline from the current blending helper. The later #1215 deliverables replace this intermediate combined-vector blending with per-repo emission_share allocation and PR/issue split spill behavior.

[anderdc]

Fold these changes into a single PR addressing #1215, not 11 slices. Closing.