TWO-24892: buyer surcharge rounding (basis + brand-driven step)

[dgjlindsay]

What

Ports the Magento buyer-surcharge-rounding feature to WooCommerce (TWO-24892, child of TWO-24739 plugin parity).

The merchant picks a rounding basis (None / Up / Down / Standard) and a rounding step from a brand-driven dropdown. The plugin relays buyer_fee_share.rounding = {step: <float>, basis: "UP"|"DOWN"|"STANDARD"} to POST /v1/pricing/order/fee. All arithmetic stays server-side — the plugin only relays the contract, mirroring Magento's Service/Order/SurchargeCalculator.

How (mirrors magento-plugin 1:1)

• brands/two.php — new available_rounding_steps brand key (default 0.10 / 0.50 / 1.00 / 5.00 / 10.00), the only source for the step dropdown. An overlay narrows the set (cf. available_terms).
• WC_Twoinc::get_rounding_step_options() — canonical two-decimal option list (number_format(…,2)) so the stored value round-trips, ascending (ksort SORT_NUMERIC); mirrors the Magento RoundingStep source model + Loader sort. Two new select fields in the offset-pricing section.
• WC_Twoinc_Payment_Terms — reads the options in get_offset_settings(); build_rounding() emits the block, omitting it for a None/unmapped basis or a non-positive step (the API requires both keys and rejects step <= 0).
• Tests — build_buyer_fee_share rounding cases (basis→API map, None/zero/negative/unmapped omission, rides alongside reference terms, ignored when offset disabled) + a Reflection test pinning the step-option producer (canonical format, sort, overlay narrowing, skip-invalid).

Semantics

• None → no rounding block sent (standard 2dp).
• Up / Down / Standard → round to nearest step (Standard = commercial half-up). The backend does the maths.

Stacking

Stacked on doug/TWO-24751-terms-chips (the only branch carrying both the buyer_fee_share builder and the brand-config layer). Rebases onto staging once TWO-24751 merges.

Out of scope (blocked, separate tickets)

• WC-ABN narrowing (0.50/1.00) → TWO-24894 — blocked: offset-pricing parity not yet ported into the ABN overlay.
• PrestaShop rounding → TWO-24893 — blocked: PS has no surcharge/offset-pricing feature yet.

Test / lint

php tests/unit/run.php green on PHP 7.4 + 8.2 (32/32); php -l clean. i18n .pot regeneration deferred to the Lokalise flow (no CI gate). Two adversarial review rounds → clean.

🤖 Generated with Claude Code

PR opened by Claude on Doug's behalf.

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Author ✍️@dgjlindsay

[dgjlindsay]

Closing: superseded. Commits fully contained in staging (integrated via #329–#339). Empty delta vs staging. Reopen if wrong. — closed by Claude on behalf of Doug