[codex] align state-protocol conformance and prep 0.1.5 release

[sorenbs]

Summary

This PR finishes the state-protocol conformance work that started with the oldValue vs old_value fix and prepares the next Streams release.

What changed

• enforce append-time validation for state-protocol streams so only valid change or control messages are accepted
• require old_value as the previous-value field and stop relying on legacy oldValue
• accept snapshot-start, snapshot-end, and reset control messages while ensuring they do not generate touch invalidations
• add runtime tests for malformed protocol payloads, control-message handling, and the old_value regression
• add a compile-time conformance check against @durable-streams/state@0.2.5
• update fixtures, demos, and docs to use the published snake_case wire format
• switch package smoke tests and release docs from npm pack to bun pm pack so release validation works with the repo's Bun package-manager pin
• bump the repo version to 0.1.5

Root cause

state-protocol streams were only loosely validated and Prisma WAL fixtures still used a camelCase oldValue field. That left the runtime short of the published Durable Streams State Protocol and forced downstream consumers to special-case Prisma WAL payloads.

Validation

• bun run typecheck
• bun run check:result-policy
• bun test
• bun test test/conformance.test.ts
• bun run test:conformance
• bun run test:conformance:local
• bun run build:npm-packages
• bun run test:node-local-package
• bun run test:bun-local-package
• bun run test:bun-server-package