IRS SOI measure_id net_capital_gains_* names two different measures (Form 1040 line 7 vs Table 1.4 Schedule D gain)

[MaxGhenis]

docs/architecture.md says that within a source_name, measure_id must distinguish different statistical measures. Two IRS SOI measure ids break that rule:

measure_id	Packages	IRS measure	Concept (after #304)
net_capital_gains_returns / _amount	historic_table_2, historic_table_2_state_broad_2022, congressional_district_2022 (and TY2023 HT2 in #295)	N01000/A01000, Form 1040 line 7 "net capital gain (less loss)": Schedule D gain, limited Schedule D loss, or distributions-only	irs_soi.returns_with_form_1040_capital_gain_or_loss / irs_soi.form_1040_capital_gain_or_loss
same ids	table_1_4	cols 37/38 (TY2022–23; 25/26 earlier), Schedule D "Taxable net gain"	irs_soi.returns_with_taxable_net_capital_gains / irs_soi.taxable_net_capital_gains

For TY2022 the US counts are 30,465,850 and 12,915,122.

#304 fixes the concepts and leaves measure_id alone on purpose. Microcosm selects SOI facts by measure_id: fiscal_targets.py maps net_capital_gains_returns to capital_gains_gross, and the capital-gains rebase control is keyed on (measure_id, filing_status, universe). Renaming either side therefore moves Microcosm's selector. microcosm#1036 and #1040 currently tell the two apart by record-set family instead.

To do. Give one side a distinct measure_id. For example, rename the line 7 pair to form_1040_capital_gain_or_loss_returns / _amount, in the same change as a Microcosm update that maps the new ids. Then drop the shared-id pin in tests/test_chronicle_soi_capital_gain_concepts.py::test_consumer_selector_survives_the_concept_rename and add a note to docs/concept-migrations.md.

Related, same packages. Historic Table 2 and the congressional-district file still give different concepts to 10 other IRS columns they both read (N/A00900, N/A01700, N/A06500, N/A25870, N/A26270). Check each against the IRS guides. For each pair that turns out to be the same IRS variable, give it one concept. Record each rename in docs/concept-migrations.md.

[MaxGhenis]

Related gap from the chronicle#304 review: tests/test_chronicle_soi_capital_gain_concepts.py finds a line 7 measure through its source_column_id or expected_column_header guard. A measure that reads N01000/A01000 by column letter alone would slip past it. The national historic_table_2 package declares 13 of its 55 measures that way today, and none of them is a capital-gain measure. Closing the gap: require every irs_soi measure that reads a CSV variable to declare source_column_id plus the header guard, as the TY2023 packages in #295 already do.