Geometry becomes a span in a single 80-byte interleaved `vertices` array
plus a span in a single `indices` array, allocated through the core's
span allocator instead of one arena array per mesh attribute. Draws are
always indexed with baseVertex; face-material overrides emit one draw per
run of same-material faces rather than one per face. Vertex data is
validated as a whole before any span is touched, same-geometry mutations
are serialised, and render signal 4 is held across a span change until
the graph reflecting it is published. Sibling arrays grow to the primary
array's capacity in one batch; reserve() takes vertex/index totals.
Amp-Thread-ID: https://ampcode.com/threads/T-01a0b2dd-db32-712d-99ce-12ce4a86d979
Co-authored-by: Heaust Azure <heaust.azure@gmail.com>
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