Clarify SIPP is public-use; only IRS-PUF is access-restricted

[MaxGhenis]

Summary

• Adds a `Licensing` section to `policyengine_us_data/datasets/sipp/README.md` clarifying that SIPP public-use data has no per-user license, agreement, or registration requirement.
• Of the six upstream microdata sources the Enhanced CPS pipeline ingests (CPS, ACS, SCF, ORG, SIPP, IRS-PUF), only IRS-PUF has a genuine access restriction.
• Makes it explicit that our HuggingFace mirror of `pu2023.csv` is a caching convenience, not an access-restriction workaround.

Why

At the 2026-04-21 meeting with Lars Vilhuber (AEA Data Editor), John Sabelhaus, and the TRACE team, John (who co-built the Sabelhaus subsynthetic beta) corrected a claim Max had been making: that SIPP requires individual user licensing. The actual SIPP vintage we use is Census public-use SIPP; the subsynthetic beta is separate. Both are effectively unrestricted. Overstating restrictions matters because it distorts which pipeline inputs genuinely warrant institutional-certification framing under TRACE.

No pipeline or ingest code changes; this is purely a docs guardrail to prevent regressions in external-communication writeups.

Test plan

• `grep` for SIPP / licensing claims across the repo — no existing misclaims to fix
• Render the README to confirm the new Licensing section formats correctly

Fixes #808.

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