Generalize the ACA-branded labels in the shared binary assignment ops

[daphnehanse11]

Summary

The generic assign_binary_from_rate / calibrate_binary_assignment / calibrate_binary_assignment_joint_targets handlers in us_runtime/source_runtime.py hard-coded "ACA binary assignment" / "ACA binary calibration" into every error message, even though they are shared manifest primitives. A failure in any non-ACA stage driving them — empirically verified with #334's person-grain medicaid_take_up stage, where dropping person_weight yielded "ACA binary calibration frame is missing required column(s)" — reported itself as an ACA failure. Closes the follow-up from the #334 review.

• Labels are now keyed to the operation kind: "US binary assignment", "US binary calibration", "US joint binary calibration", matching the file's existing "US ratio computation" / "US support clip" naming. The shared helpers (_binary_calibration_value_state, _preserved_true_mask, _first_available_target_table, _group_columns, _domain_mask) take a label keyword so each handler reports its own name. The _ACA_ prefix is dropped from the corresponding parameter-key frozensets. aggregate_person_to_tax_unit keeps its ACA labels — it is genuinely ACA-specific (Marketplace coverage aggregation) and only the ACA stage uses it.
• Draw salts are deliberately unchanged. aca:{output}, calibrate:{variable}, and joint-calibrate:{variable} are byte-identical, and the aca: prefix is now commented as frozen: existing ACA releases are bit-reproducible against it. The exact-seeded-outcome assertions in the existing ACA tests pass unchanged, which locks this in.
• The calibrate handlers' silent stable_tax_unit_draw default is removed: draw is now a required parameter (the assign handler already required it), and the ACA manifest's three calibrate ops state the previous default explicitly — bit-identical behavior. A future person-grain stage that omits the param now fails loudly instead of silently minting positionally-keyed (not identity-stable) draws under a tax-unit column name. _stable_draws gained a docstring stating its tax_unit_id-vs-positional-index keying and that stages at other grains should mint their own identity-keyed draw column.

Interaction with open PR #334 (medicaid_take_up)

• Materialize Medicaid take-up via anchored count-calibration to CMS state snapshots #334 appends a new medicaid_take_up stage to source_stages.json; this PR edits only the existing ACA stage's calibrate ops. The hunks are disjoint, so the two merge cleanly in either order.
• Materialize Medicaid take-up via anchored count-calibration to CMS state snapshots #334's medicaid ops already pass an explicit draw: "stable_person_draw" param and pre-compute the draw column identity-keyed at person grain, so they comply with the stricter required-draw contract as written.

Tests

• New: a person-grain reproduction of the mislabeled error (asserts the message names "US binary calibration" and contains no "ACA"), required-draw errors for both calibrate handlers, and a person-grain calibration happy path with a declared draw column.
• Full packages/populace-build suite passes (884 tests); ruff check/format clean on the changed files.

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