feat(format): expose transport field validity

[shipyard-local]

Summary

• add a compact typed validity mask to ProcessContext and HostTransport
• migrate CLAP transport decoding and playhead diffs to validity-aware semantics
• preserve existing behavior for adapters that have not migrated yet

Validation

• focused Release tests: 380 assertions across 43 cases
• no-exceptions build/tests
• local pre-push gates and diff-coverage gate clean
• strict architecture review: no must-fix findings for this slice

Scope

This is an independent transport-boundary hardening slice and does not depend on the timeline device-placement stack.

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🔎 Provenance

Agent	codex
Machine	m3
Tab	Timeline v2
Directory	/Volumes/Workshop/Code/pulp
Session	019f76f4-ca64-7c11-bfab-f5e4760ad402

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codex resume 019f76f4-ca64-7c11-bfab-f5e4760ad402

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_{stamped 2026-07-20 05:26 UTC}