[HughRunyan]

Stacked on #91. Merge #91 first; this PR then targets main.

What

City.dst_baseline_blank takes population_series, the same way load_csv_new does since #91: the country's population by year from country_population_trajectory, which WasteMAP's map build stores. A Custom Location's waste, and its scenario's, grow along it instead of along SWEET's own pops_yearly.csv. The tonnage still comes from the population passed in; the two growth rates a person reads (growth_rate_historic / growth_rate_future) are still the country's, as in load_csv_new. Without the argument nothing changes.

As a WasteMAP maintainer, I want the city DST's Custom Location to grow along the same stored country trajectory as everything else, so that a SWEET pin change can't make it grow differently from the build behind it.

Does any number move?

Checked every country SWEET knows (250; a 1,000,000-person Custom Location; baseline emissions and waste by year), SWEET's table vs the stored trajectory (after #91's 67e21ca, which keeps the UN's fractions of a person):

• 12 places with fallback rates (Antarctica, …): bit-identical.
• The other 238: within 5.9e-16 (median 3.6e-16). SWEET divides each year's population by the 2022 figure, which can land one unit in the last place away. Growth rates within 2.2e-16.

Acceptance criteria

• dst_baseline_blank(..., population_series=s) grows the baseline and scenario waste along s / s[2022].
• The country's own trajectory reproduces the old Custom Location to 1e-14; a fallback-rate country's exactly.
• The two growth rates stay the country's.
• No caller that omits the argument changes.

Tests: 3 new in tests/test_population_trajectory.py (fail before the change). Full suite: 572 passed.

Paired WasteMAP PR: RMI/WasteMAP#890. Same change on the ADST branch: #95 (with RMI/WasteMAP#891).

Definition of Done

Acceptance criteria met · tests pass · changelog updated · reviewed & merged.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The optional override follows existing growth paths, has targeted regression coverage, and has no identified blocking issues.

No open issues 🎉

What changed in this PR

Extends #91’s stored population trajectories to Custom Locations, keeping their waste growth aligned with WasteMAP’s stored build data.

Changes:

• Adds optional population_series support while preserving population-based tonnage, reported growth rates, and default behavior.
• Tests baseline growth, fallback equivalence, and scenario growth.
• Documents the feature in the changelog.

File	Description
tests/​test_population_trajectory.py	Tests stored trajectories and Custom Location behavior.
SWEET_python/​city_params.py	Adds trajectory helpers and optional stored-series growth.
changelog/​2026-10.md	Documents trajectory support and numerical compatibility.

_{Review effort: Balanced}