Several downstream crates (acpid for SMBIOS scanning, redox-drm, GPU
drivers) hold the physmap error in a map_err adapter. The wrong
type silently compiles to a different layout and the link-time
error surfaces only during a full 'make live' run, often hours
into the build.
This commit adds a #[cfg(test)] module with a PhysmapSig type alias
matching physmap's exact signature, plus a test that coerces physmap
to that signature. If physmap's error type drifts (e.g. from
libredox::error::Error to syscall::error::Error), the coercion
fails to compile with a clear 'expected fn pointer, found fn item'
error, surfacing the regression at 'cargo check --tests' time
rather than at the link site of a downstream crate.
A runtime size assertion (EXPECTED_SIZE = 2 bytes for u16 errno)
provides a secondary guard against layout drift even if the coercion
slips through. Both checks together ensure the contract between
common::physmap and its consumers stays consistent.
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