Add opt-in decimal quantity support

[sultann]

Summary

Supersedes #261 (closed). Adds optional decimal quantity support to the plugin using WooCommerce's official woocommerce_stock_amount filter extension point. Feature is off by default so existing installs are unaffected.

How it works

When the "Allow decimal quantities" setting is enabled:

1. Plugin::bootstrap() removes WC's default intval filter on woocommerce_stock_amount and registers floatval instead. WC then funnels every quantity (add-to-cart, cart updates, checkout, order items, emails, REST) through floatval — no $_POST hijacking, no static state, no reentrancy bugs.
2. All comparison / math / display sites in Cart.php branch on wc_is_stock_amount_integer() (WC 10.1+, with a function_exists fallback that assumes integer mode on older WC).
3. Helpers in functions.php centralize the branching: wcmmq_is_integer_qty(), wcmmq_format_qty(), wcmmq_is_valid_step().
4. Admin quantity inputs get step="any" so decimals can be entered when the feature is on.

Why not "just widen casts to float"

Two traps the previous PR (#261) fell into:

• WC's intval filter wins by default. add_filter('woocommerce_stock_amount', 'floatval') alone runs after WC's intval, which has already truncated the decimal. Data loss. Must remove_filter first.
• Integer math hides in many places. absint(), % (PHP forces int operands), and number_format() without precision all silently strip or round decimals. With step=0.5 and max_qty set, absint(0.5) is 0, and $max % 0 fatal-errors on PHP 8+. These needed direct replacement with wc_is_stock_amount_integer() branches.

Pro plugin coordination

Pro plugin PR submitted in parallel: https://github.com/pluginever/wc-min-max-quantities-pro/pull/new/feature/decimal-quantity. These must merge and ship together — the free plugin widens limit storage to float, and Pro's Cart.php was casting back to int via the wc_min_max_quantities_product_limits filter. Shipping only one would break the other.

Files changed

File	Change
includes/Plugin.php	Gated remove_filter(intval) + add_filter(floatval) in bootstrap()
includes/functions.php	Added 3 helpers, widened limit casts to (float)
includes/Cart.php	Replaced absint()/%/number_format() with helper calls at 20+ sites
includes/Admin/Settings.php	New wcmmq_decimal_quantities checkbox + warning copy + step="any" on qty fields
includes/Admin/MetaBoxes.php	step="any" on product step field

Test plan

Setting OFF (default) — must behave identically to v2.2.9

• Fresh install, default settings → integer quantities enforced as before
• Integer min/max/step on a product → existing validation behavior
• Cart/checkout flows unchanged
• wc_is_stock_amount_integer() returns true

Setting ON

• Enable setting → confirm WC input inputmode becomes decimal (WC handles this automatically)
• Set product min=0.5, max=5, step=0.5 → add 2.5 to cart, no fatal, validation passes
• Set step=0.5, max=3.5 → divisibility check passes (canonical fmod + epsilon)
• Set step=0.3 → fmod(3.6, 0.3) case, precision tolerance works
• Cart-level min_qty=2.5 → error message shows "2.5", not rounded
• Exceed max_qty=2.5 with 3 items → error uses wcmmq_format_qty(), shows "2.5"
• Variable product with decimal variation step → quantity input respects decimal step
• Store API block cart → min/max/multiple_of flow through correctly
• Order completion + order emails → quantities preserve decimals

WC version fallback

• Force function_exists('wc_is_stock_amount_integer') to false (simulate WC < 10.1) → helpers fall back to integer mode, no fatals

Pro plugin coordination

• Both branches active → role-based limits with decimal values work
• Category-level decimal limits work end-to-end
• Variation override with decimal values respected