Resolve parameter nodes at instant on first read

[MaxGhenis]

Draft: measurements on policyengine-us and policyengine-uk are running; this description gets the before/after numbers when they finish.

Problem

ParameterNodeAtInstant.__init__ built every descendant up front. Every instant a tax-benefit system is asked about therefore materialised the whole parameter tree, and ParameterNode._at_instant_cache (plus TaxBenefitSystem._parameters_at_instant_cache) keeps it for the system's lifetime.

Measured on policyengine-us (132,396 parameter descendants) on master b78b0ba:

• each instant costs about 34,000 node objects, 16 to 33 MB, and 4 to 5 s on a loaded host, on every system;
• a household on a reform system added 349 MB, most of it the reform system's own at-instant trees.

A YAML run touches every year it tests and every month of a monthly variable, so the shared system and each reform system all pay this many times over. It was the third cause of the 118 GB policyengine-core test runs on 2026-10-02, next to the test-runner causes fixed in #569; the two PRs are independent.

Change

A node at instant resolves a child the first time it is read (__getattr__, __getitem__) and keeps it. It resolves all of its direct children only when something needs all of them: _children, iteration, repr, dir, vectorial indexing. A resolved child is a plain instance attribute, so hot reads cost what they did.

Invariants (tested)

• For any instant and any order of reads (attribute, item, iteration, _children, dir, repr, vectorial indexing, names that do not exist), every read returns what the up-front build held, and a node read in full equals the up-front build, including child order. test_parameter_node_at_instant_property.py (Hypothesis) checks this against a reference that walks the ParameterNode tree directly, and it passes on master too.
• A child resolves once; its identity is stable.
• Unknown names raise the same ParameterNotFoundError / KeyError as before.
• Intended difference: a node at instant held across an in-place change to its parameter tree reads, for a child not yet read, the value at the time of that first read; the old node held the value at construction. A node requested after the change behaves as before. (test_a_child_not_read_before_an_update_reads_the_update)

Tests

• uv run pytest tests/core/parameters tests/core/parameters_fancy_indexing tests/core/test_parameters.py tests/core/test_tracers.py tests/core/test_reforms.py
• The whole core suite passes locally.

🤖 Generated with Claude Code

[MaxGhenis]

Measurements of this PR combined with #569 on the two policyengine-us CI batches that are failing on 16 GB runners: see #569 (comment). Together they take the NY/OH/OK batch from 16.3 GB (3.32.11) to 7.0 GB and the USDA batch from 17.1 GB to 6.4 GB peak footprint.