Add tree-shaped parameter loader mirroring Python (#50)

[vahid-ahmadi]

Addresses #50.

Scope

Lands a tree-shaped parameter loader (ParamTree) that mirrors the Python policyengine-uk directory-tree parameter system. This is additive: it does not delete the existing flat year-files (parameters/YYYY_YY.yaml) or the hand-coded Parameters struct, and it does not change any existing consumer. The new loader coexists with the existing system so nothing breaks and everything still compiles.

New module src/parameters/param_tree.rs:

• ParamTree::from_dir walks a directory tree of YAML parameter files and parses each leaf into a typed Node tree. Directories become interior Subtree nodes; .yaml/.yml files become leaves keyed by file stem.
• Supports the three Python parameter node types:
  • values: — date-keyed scalars
  • brackets: — list of { rate, threshold }, each date-keyed
  • scales: — synonym for brackets:, handled identically
• Path resolution mirrors Python: tree.get_at("gov.hmrc.income_tax.rates.basic_rate", 2025) returns the value in force at that date, using step-function semantics (latest entry on or before the requested date). null entries clear a value within a window.
• Forward auto-uprating: a value requested beyond its last declared year is projected forward using the index declared in metadata.uprating (e.g. gov.indices.cpi), scaling the last-known value by index(requested) / index(base). With no index the series is held flat.
• A small fixture parameter tree under tests/fixtures/parameters/gov/ (income tax rates/personal allowance, UC standard allowance, Scottish LBTT brackets, CPI index) gives the loader real YAML to parse.

Tests

8 unit tests covering: value-by-date resolution (including before-first-entry → None), the step-function picking the latest on-or-before entry, directory path lookup, bracket/scale parsing and resolution, forward uprating projection beyond the last declared year, holding flat without an index, the scales: synonym, and null-within-window clearing.

cargo build and cargo test both pass (198 existing tests + 8 new, 0 failures).

Remaining

This PR lands the loader infrastructure only. Migrating the compute_* consumers in src/variables/ off the hand-coded Parameters struct onto path-keyed ParamTree lookups (and copying the authoritative gov/... tree from policyengine-uk) is the follow-up for #50 — out of scope here because it would touch every consumer at once.

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