feat!: make property tax a list of match/formula rules

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The tax was a global default-formula plus exempt-plot-threshold, with rules as an optional per-property override. Scoping the tax to a subset of plots was not expressible: rule predicates have no negation, and a rule match removes a plot from the aggregate count, so the plots you want counted cannot be shielded from a catch-all.

The tax is now the rules list and nothing else. Each rule pairs a tag predicate with the formula charged to the plots it claims:

• every plot falls to the FIRST rule matching its tags; a plot matching no rule is untaxed and counted nowhere
• each rule's formula is evaluated ONCE per owner, with bound to that owner's plot count in THAT rule, not their total holdings
• exempt-threshold moves onto the rule, so each bracket carries its own
• charges are summed and floored to the cent

A server-wide tax is a rule with no match. Rules are aggregate rather than per-property, which loses nothing: a flat per-plot rate is 10 * <plots>. A rule whose formula fails to parse is dropped and its plots fall through to the next match, so a typo under-charges rather than charging something unintended.

BREAKING CHANGE: default-formula, exempt-plot-threshold and TaxSettings.DEFAULT_FORMULA are removed. An un-migrated taxes.yml has its rules key filled from the shipped defaults on startup, switching that server to the packaged rule. rules: [] does not disable collection — Configurate treats an empty list as absent, so the default returns on the next restart; enabled: false is the off-switch.

Tests: 18 policy tests cover scoping, per-rule counting and exemption, rule ordering and drop-on-parse-failure. A new TaxesConfigTest loads the packaged taxes.yml as the plugin does and pins that it deserializes, that its formulas compile, that it charges the figures its own comments quote, that every tag it names exists in region-tags.yml, and both startup-merge behaviours above.