Maintenance docs

Procedures for keeping the vendored dependencies and the generated artefacts of this fork up to date. Same rule as docs/internals/rendering/: a procedure that had to be rediscovered gets written down here in the SAME commit as the work — exact commands, the versions they were run with, what breaks when a step is skipped, and the dead ends.

Page	What it covers
valhalla-upgrade.md	Merging a new upstream Valhalla release into mbtiles-support, regenerating protos / locales / the tz database, and the fork patches that must survive
mac-catalyst.md	Why the Catalyst slices are a macOS project in disguise, what that breaks at link time, and what the build gives up to work around it
angle.md	Rebuilding the vendored MetalANGLE static slices, the strip settings that make them shippable, and the one fork patch to re-apply
flatten-submodules.md	Moving libs-external / libs-massif in-tree while keeping the third-party forks as submodules — the history graft, the .gitmodules hoist, and what has to be fixed by hand
api-typings-chain.md	From a .i declaration to an app's autocompletion: what generates what, the --defines trap, and how to tell which link is stale
branding.md	The one mark every logo, favicon, social card and launcher icon is generated from — and why the Android safe zone is smaller than it looks
web-build.md	Building the SDK under emscripten: the web/native platform layer, the cross-origin-isolation and main-thread-GL rules, where the site's style preview gets its wasm, and what the wasm build does not carry yet