Support threshold uprating in scale parameters

[MaxGhenis]

Fixes #390.

Problem

Scale (marginal-rate / amount) parameters only uprated thresholds through two placements:

• the per-leaf form brackets[i].threshold.metadata.uprating, and
• a scale-wide metadata.uprating combined with metadata.uprate_thresholds: true.

Every other placement was silently dead, so bend points and indexed brackets froze at their last explicit value:

• bracket-level brackets[i].metadata.uprating (the amount stayed frozen — policyengine-us#8814),
• the issue's own brackets[i].metadata.threshold.uprating sub-key form,
• a scale-level metadata.threshold_uprating dedicated to thresholds.

(The controlled experiments confirming which forms worked are in the #390 comments.)

Change

ParameterScaleBracket.propagate_uprating now resolves an uprating entry for each component leaf, and ParameterScale.propagate_uprating passes through the scale-level threshold_uprating. Resolution order per component, highest priority first:

1. the component leaf's own metadata.uprating (the existing per-leaf form — unchanged);
2. a bracket per-component override, e.g. brackets[i].metadata.threshold.uprating / brackets[i].metadata.amount.uprating;
3. a bracket-level default — brackets[i].metadata.uprating for the value side (amount/rate/base/average_rate), and for the threshold either brackets[i].metadata.threshold_uprating or the bare bracket uprating when brackets[i].metadata.uprate_thresholds is set;
4. the scale-level defaults — metadata.uprating (gated by uprate_thresholds for the threshold) and the new metadata.threshold_uprating for a threshold-only index.

This matches the issue's proposed solution: threshold_uprating in scale metadata, uprating in individual threshold metadata, and both (individual overriding the scale default).

Semantics I had to interpret

The issue and its comments show two shapes for bracket-level metadata: a bare brackets[i].metadata.uprating (expected to uprate the amount, per policyengine-us#8814) and an explicit brackets[i].metadata.threshold.uprating (to uprate the threshold). I read a bare bracket uprating as value-side only — mirroring the existing scale-level rule where a bare uprating never touches thresholds unless uprate_thresholds opts them in. The threshold is therefore reached only via the threshold sub-key, a bracket/scale threshold_uprating, or an uprate_thresholds flag. If a bare bracket uprating were instead meant to hit the threshold too, that is a one-line change — flagging the assumption explicitly.

uprate_parameters already reaches every bracket leaf through get_descendants (locked with a dedicated test), so no change was needed there; the gap was purely in propagation.

Tests

New tests/core/parameters/operations/test_scale_threshold_uprating.py (13 tests): the three previously-working forms are locked as regressions, each newly-supported placement is covered, plus threshold_uprating combined with a distinct amount index, per-leaf override of the scale default, threshold-index leaving amounts frozen, and get_descendants bracket-leaf coverage. Also verified the exact PIA example from the issue body uprates (bend point 1226 → 1284.38 in 2026).

Checks run

• uv run pytest tests -m "not smoke" → 599 passed, 1 skipped, 1 xfailed.
• policyengine-core test on the bundled country template → 39 passed.
• ruff format (clean) and ruff check (passed).
• Not run: the -m smoke suite (requires network / GitHub microdata token).

Note on overlap with #506

Independent of the sibling PR #506 (issue #505). #506 renames the bracket's component-key tuple from _allowed_keys to _component_keys and adds the common keys for validation; this PR keeps iterating _allowed_keys. Whichever merges second will have a trivial rename conflict in parameter_scale_bracket.py.

🤖 Generated with Claude Code

[MaxGhenis]

Fable review: TDD reproductions of every broken form, four-level resolution precedence with the value-side/threshold distinction preserved (bare uprating never silently hits thresholds, consistent with scale-level semantics), the documented interpretation of bare bracket uprating flagged for the issue author, and the post-#506 rebase iterates component keys correctly. 608 core tests + 39 country-template green; the issue-body PIA example verified uprating (1226 → 1284.38).