`external 'c' name 'strlen'` now records the bare library name on the
declaration (TMethodDecl.ExternalLib) and hoists it into the owning
unit/program's LinkLibs set, so the link layer can expand it to a
-l<name> dependency. The library clause is optional and parsed before
the existing `name '...'` clause, so `external name 'x'` and bare
`external 'c'` both still parse.
TUnitInterface.LinkLibs is serialised in the .bif META block (one
EncodeStringList after ImplUsedUnits, before HasInitialization) so a
unit's external-library dependencies round-trip across separate
compilation. IFACE_VERSION is bumped 6 -> 7: the META layout grew, so
a v6 reader must reject these .bif and recompile (without the bump a v6
binary would misparse the extra field and read past the END marker).
Tests: a parser test asserting `external 'c' name 'strlen'` populates
ExternalLib + the program's LinkLibs; a .bif round-trip test for
TUnitInterface.LinkLibs; the magic/version assertion updated to v7; and
the existing e2e strlen case. bif-coverage.status records
TUnitInterface.LinkLibs as serialised.
c1f14b5c78