V48 Gate 3 (specification-only): Final rebuild-alone full-stack canon and close

Expand BITCODE_SPEC_V48.md Gate 3 law into G3-1…G3-15 covering executions
schemas, deposit APIs, Inline/VercelSandbox hosts, SDIVF agent/tool/prompt/
store/telemetry contracts, option full-stack stats, /deposits UI, fail-closed
errors, env rebuild checklist, and source map — sufficient to rebuild deposit
MVP from this SPEC alone. Mark Gate 3 closed in notes, parity completion, and
QA ledger; ops smoke remains optional.

Author: geraldarthurdavis