v0.1.36 - KV-friendly prompt assembly hotfix

This hotfix fixes a real performance regression in long local-model sessions: Successor now preserves a KV-cache-friendly request shape instead of quietly rewriting earlier prompt bytes as the conversation grows.

What changed

• made prompt assembly append-only in the places that matter for local cache reuse
  • runtime-tail context is now always appended as its own synthetic turn instead of being merged into the latest real user message
  • adjacent same-role API messages now preserve their original boundaries instead of being merged together later
• stabilized native tool-call serialization
  • assistant tool_calls[].function.arguments now use canonical JSON ordering so identical tool payloads serialize to identical bytes
• added better cache-shape diagnostics
  • turn traces and /perf now record request prefix hash, full request hash, and tail kind alongside the existing stable-system hash and llama timing data
  • this makes it possible to distinguish harness-side request-shape churn from provider-side cache dropout

Verification

• ruff check src tests
• PYTHONPATH=src pytest -q
  • 1277 passed in 25.56s
• live llama.cpp KV verification
  • before fix: repeated late-turn cache_n=0, median first-token latency ~34s, suspected KV misses across many turns
  • after fix: cache_n grows turn-to-turn, hit ratio stays roughly 0.8–0.94, and first-token latency stays sub-second in the extended local-model session