docs(runtime): sync all docs with implementation, add SDK encoding helpers

[simonovic86]

Summary

Sync documentation across all runtime modules to reflect current implementation state through Phase 5 (Hardening) and Task 15. Add new SDK encoding helpers for fixed-size checkpoint serialization.

Changes

Documentation Updates

• CLAUDE.md: Update phase/task status, add make demo target, clarify adaptive tick loop (1 Hz default, 10ms fast path), expand architecture section with new internal packages
• IMPLEMENTATION_STATUS.md: Update checkpoint format to v0x04 (209-byte header), mark lease-based authority (Task 12) and signed lineage (Task 13) as implemented, note divergence detection requires Task 15
• DEVELOPMENT.md: Update Go requirement to 1.25+, increase cyclomatic complexity limit to 20, expand directory structure with new packages (authority, eventlog, hostcall, inspector, pricing, registry, replay, runner, settlement, simulator, timeline, wasmutil, budget, identity, lineage, receipt)
• AGENT_LIFECYCLE.md: Document 5 WASM exports (add return value to agent_tick), detail v0x04 checkpoint header (209 bytes with lease/lineage metadata), update cost calculation to nanosecond precision, provide v0x04 counter example
• ARCHITECTURE.md: Update hostcall table with implementation status, clarify metering precision and overflow guards, detail full v0x04 checkpoint structure with lease and lineage fields
• IMPROVEMENTS.md: Mark improvements feat(agent): checkpoint v2 with persisted tick number #3–fix(runtime): enforce manifest policies and fix replay timeout safety bug #10 as complete (Observation-Weighted Retention, Multi-Tick Chain Verification, Replay Failure Escalation, Adaptive Tick Rate, SDK Checkpoint Serialization, Event Log Allocation Optimization), describe implementation details for each

SDK Enhancements

• encoding.go: Add Raw(b []byte) method for fixed-size fields without length prefix, add FixedBytes(n int) decoder, add ReadInto(dst) convenience method for array fields
• encoding_test.go: Add 8 new tests: TestEncoder_Raw_RoundTrip, TestDecoder_ReadInto, TestDecoder_FixedBytes_ShortRead, TestDecoder_ReadInto_ShortRead, TestEncoder_Raw_Empty, TestRaw_vs_Bytes_SizeComparison plus helper test

Notes

• No code logic changes; purely documentation alignment + SDK API additions for fixed-size encoding
• Raw and FixedBytes improve ergonomics for checkpoint serialization patterns like [16]byte arrays
• Full test coverage for new encoding helpers

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