feat(plugins): bundled era-based tax rate tables (CH, EU, UK, US) + update-tax-rates skill

[pozylon]

What

Moves tax rates out of code into bundled, era-based JSON tables — versioned, reviewable, and historically complete — and adds aligned tax plugins for the EU, the UK and the US next to the existing Swiss pair, plus an update-tax-rates maintainer skill that verifies/updates all bundled tables against official sources.

Targets v4.8.x. Rates are deliberately not fetched at runtime: tax rates are legally binding, so every change must be a reviewable diff, and the applicable rate follows the supply date — which requires the full era history, not "the current rate".

Commits (atomic per plugin)

1. shared — eraRates (compile/sort {validFrom, rate} eras, resolve by reference date, clamp before first era), applyTaxRateToTaxableRows (the common adapter tax math: gross rows get tax extracted via offset+tax rows, net rows additive), resolveDeliveryLocation (delivery-address → billing-address → order-country, incl. regionCode; isDeliveryAddressInCountry now delegates).
2. CH — SwissTaxCategories reads ch-tax-rates.json (emitted to lib/ by tsc via ESM JSON import attributes). Public API unchanged. Behavior fix: 2011–2017 orders were priced at the 2018–2023 rates (7.7/2.5/3.7%); the table resolves the legally correct 8.0/2.5/3.8%. History back to 2001, earlier dates clamp.
3. EU — eu-tax-rates.json with per-country era histories for all 27 member states (verified against the EC's published rates; incl. DE 2020 temporary cut, LU 2023, CZ/EE 2024, SK/RO/EE 2025, FI/LT 2026). Destination country picks the table; eu-tax-category:<name> product tag or provider config picks the category, defaulting to standard (unknown categories fall back to standard, never zero). Greece resolves as GR and EL. OSS/reverse-charge out of scope.
4. UK — standard (17.5 → 15 → 17.5 → 20 incl. the 2008–2010 crisis cut), reduced (8 → 5), zero; UK VAT area = GB + Isle of Man; verified against gov.uk / Commons Library history.
5. US — statewide base sales-tax rates for 50 states + DC (era history for LA 2025, NM 2022/23, SD temporary cut, DC's enacted 2026-10-01 increase). State from the delivery address regionCode; unknown state applies no tax (logged) instead of a made-up fallback; us-tax-category:exempt tag / provider config opt-outs. Documented approximation: no county/city add-ons, category taxability, nexus or sourcing — full US compliance needs a tax service.
6. skill — .claude/skills/update-tax-rates: per-region official sources (ESTV/Fedlex, EC TEDB, gov.uk, state DORs), cross-check rule, append-only eras, never guess, tests updated with each change, human review before commit.

Verification

• @unchainedshop/plugins: 136/136 unit tests pass, including new suites for era histories, exact effective-date boundary switches, tag/provider category resolution, and per-file JSON-consistency checks (ascending eras, plausible rates, source/verification metadata)
• tsc -b clean; all four JSON tables emitted to lib/ and loadable from the compiled ESM output
• eslint + prettier clean

All rate data was researched with per-country/state source citations; figures that could not be verified against an official source were excluded rather than guessed. Effective dates for long-standing US rates use the source table's as-of anchor date (documented in the research; single-era states resolve identically for all past dates via era clamping).

Related: #726

7. docs — one page per new adapter following the Swiss tax doc template, pricing-plugin index extended, Swiss pages updated for the era-based tables (incl. the corrected 2011–2017 / 2001–2010 rate columns), and a prominent statewide-approximation warning on the US pages.