US: store years_since_us_entry from PEINUSYR in the national pool

changelog.d/776-years-since-us-entry.added.md

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+Store `years_since_us_entry` on ASEC person rows from the CPS `PEINUSYR` year-of-entry code, so the engine's five-year bar (8 USC 1613), the Missouri SSP waiting period and the Washington TANF and RCA windows read a measured clock instead of the default of 5 (#776). The immigration stage now owns the column. For the foreign-born it is the time period minus an arrival year drawn uniformly within the person's `PEINUSYR` band, clipped at zero. The band is read in the codebook of the person's own ASEC vintage, because Census re-bins the top codes every year. It is bounded below by birth year, and the draw uses a keyed draw with its own salt, so the status labels are byte-identical. For the US-born the clock is their age. A foreign-born code with no reviewed codebook interval, or a vintage with no reviewed codebook, is refused. In the national pool the column joins the immigration operator's output family, so ACS rows receive it through the same chained transfer as their status labels, on the donors' integer support. The composition summary reports the weighted entry-clock distribution by status, including the LPR share under five years, and the composition gate fails a stored clock that is incomplete, negative or at the engine default for every non-citizen. A base labelled before this change passes through without the clock. The catalog gives the clock the duration unit `years`. The ACS local lane's immigration stage (#1052) reuses the stage's column and engine-default constants and keeps a donor's stored clock, so its `PEINUSYR` bin-year fallback applies only to donors labelled before this change.