Carve the Codex decoder into @codeburn/core/providers/codex as a pure function
over supplied rollout records with EXPLICIT, JSON-serializable state
(CodexDecodeState): the running cumulative token counters, fork-replay cutoff,
mid-turn accumulators, id/turn bookkeeping, and the cross-file dedup memory
(seenKeys). Decoding a corpus in one pass equals decoding it in any number of
passes when the state threads between them (the resume invariant).
Cost leaves the decoder: the rich calls are cost-free and the CLI prices them
through the established estimated-cost seam (costBasis: 'estimated'), retiring
both Phase-0 residual calculateCost sites. The CLI codex provider is now
discovery + file streaming + cache I/O + pricing.
codex-results cache bumped to v8 (one deliberate bump): it persists the decoder
end-state blob + a byte offset + host-priced calls, so a grown rollout resumes
from the stored state and decodes only the appended bytes. Lossless — rollout
files are durable, so the one-time re-derive rebuilds identical data.
Adds a codex path to the content-smuggling guardrail and the import-smoke
subpath set. Byte-parity verified against a frozen 826-file corpus (codex-only
and all-providers, rel_tol 1e-9); perf within ±1.5% cold / ±0.5% warm.
