[HughRunyan]

Merge #90 first. This PR is based on #90's branch, so its diff is only the change below. Once #90 is merged into am-waste-flow, GitHub retargets this PR there. I'll then merge am-waste-flow back in so the diff stays clean.

Output

The same change as #91 (into main), for SWEET's advanced DST branch. It is one cherry-picked commit plus its changelog links:

• city_population_trajectory and country_population_trajectory give a city's or a country's population in people for every model year, and what the shape comes from ("UN city rates", "country curve", "country fallback rates").
• load_csv_new(..., population_series=) grows a city along a stored trajectory.

Why

WasteMAP's advanced DST now grows a city's waste along the same stored trajectory the city DST uses. Its city prefill serves the trajectory, and the API passes it to load_csv_new. WasteMAP's am-waste-flow builds with this branch of SWEET, so the API there needs these functions. The WasteMAP PR that uses it pins this branch until this merges.

Numbers

No number moves in SWEET: nothing calls the new argument by default. See #91 for why a stored trajectory reproduces SWEET's own growth to ~1e-15 rather than to the bit.

Acceptance criteria

• Same code as A place's population by year can be worked out once and handed back to SWEET #91; the changelog entry is identical, so a later main → am-waste-flow catch-up merges cleanly.
• pytest -m "not integration": 604 passed on this branch (with The city-level advanced DST sends waste no listed landfill takes to other landfills #90).

Definition of Done

Acceptance criteria met · tests pass · changelog updated · #90 merged first · reviewed & merged.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

No blocking issues were identified; existing defaults are preserved and focused regression coverage accompanies the changes.

No open issues 🎉

What changed in this PR

Adds stored population trajectories to SWEET’s advanced DST branch, allowing WasteMAP to reuse population growth curves without changing default behavior.

Changes:

• Adds city and country trajectory helpers and optional population_series loading.
• Adds regression tests for trajectory generation and reuse.
• Documents the feature and numerical precision.

File	Description
tests/​test_population_trajectory.py	Tests trajectory shapes, fallbacks, and loader integration.
SWEET_python/​city_params.py	Adds trajectory helpers, stored-series support, and shared fallback constants.
changelog/​2026-10.md	Documents stored trajectories and related PRs.

_{Review effort: Balanced}