feat(producer): version distributed plan protocol

[jrusso1020]

Summary

• write an explicit v1 distributed-plan protocol descriptor without changing plan hashes or rendered pixels
• accept descriptor-less historical v1 plans, while failing closed on partial, malformed, or unknown descriptors
• validate the descriptor before chunk and assembler workers inspect v1-specific artifact paths
• expose per-role planner/chunk/assembler capabilities that can represent a v2 planner with dual v1/v2 readers

Why

The current monolithic plan layout needs to evolve before PLAN_TOO_LARGE can be removed safely. Rolling out a new layout without an explicit protocol risks an old assembler silently consuming only the v1 artifacts it understands (for example, omitting future externalized audio). This PR is the pixel-neutral compatibility prerequisite; it does not introduce Plan v2 or enable any new render path.

Compatibility and rollout

• descriptor absent: treated as legacy plan-dir-v1
• known complete v1 descriptor: accepted; unknown optional metadata is ignored
• partial/malformed/unknown descriptor: typed PLAN_PROTOCOL_UNSUPPORTED before layout-specific reads
• capabilities advertise planner.produces, chunk.accepts, and assembler.accepts independently, plus descriptor-less legacy support only for reader roles
• a future v2 rollout must still use an explicit history-recorded protocol choice, a distinct v2 locator/prefix, and build-ID pinning or a dedicated canary queue; no mixed-protocol render is permitted after the first chunk

Validation

• 20 focused protocol/public-reader tests
• 41 distributed plan tests
• producer typecheck
• changed-file oxlint and oxfmt
• test classification and tracked-artifact checks
• pre-commit checks
• independent review completed after fixing validation-order and dual-reader capability blockers