Surface rename of the signed integer family: s1..s64 become i1..i64
(u1..u64, usize, isize unchanged). 'string' keeps the s-prefix arm in
name classification; width parsing moves to the i-prefix arm next to
isize.
Internal TypeId tags follow the surface (.s8/.s16/.s32/.s64 ->
.i8/.i16/.i32/.i64), as do mono-key mangle fragments (ptr_i64,
tu_i64_bool) and all display/diagnostic formatting (i{d}).
Migrated in the same sweep: stdlib + examples + issue repros + FFI C
companions (shared symbol names like ffi_id_i64), expected
stdout/stderr/ir snapshots, specs.md, readme.md, CLAUDE.md/AGENTS.md,
implementation_plan.md, docs/, issue writeups. Vendored stb_image and
historical flow state left untouched.
zig build test: 426/426; examples suite: 595/595.
When a module declares `A :: B; B :: u64;` and both a flat import and a
namespaced import export `B :: u8`, the flat import's B was discovered by
flatTypeAuthorCount before the own B :: u64 was processed — binding A to
u8 and silently truncating values.
Fix: ownConstDeclIsPendingAlias guard added to selectNominalLeaf between
the own-alias check and the flat-import walk. If the querying module has
an own const_decl for the name that is not yet in type_aliases_by_source,
return .pending so the forward-alias fixpoint resolves it correctly.
Regression: examples/0830-modules-flat-ns-same-name-forward-alias.sx
(x : A = 300 prints 300, not 44). 541/541 tests pass.
