docs: distinguish budget authority from payment rails

[amavashev]

Summary

• define a normative boundary between Cycles budget authority and payment-rail operations
• clarify the README lifecycle wording for reserve, commit, and release
• bump the runtime revision to 0.1.25.15 and document the semantic clarification
• regenerate the merged runtime artifact

Why

Payment integrations can otherwise conflate a Cycles budget reservation with a payment authorization, a Cycles commit with capture or settlement, and a Cycles release with a void or refund. These operations belong to different layers and do not prove that funds moved or were reversed.

Post-commit reversal and reconciliation semantics are tracked separately in #133.

Impact

Documentation-only semantic clarification. No endpoint, schema, or wire-format change; semantic_base remains 0.1.25.

Validation

• Spectral lint across all canonical source specs and merged artifacts: 0 errors (existing warnings remain)
• changelog pointer/version validation passed
• merged artifact generation is idempotent
• git diff --check passed