compileCWithEmcc now probes/saves .sx-cache/c-<key>.o with the same
content key as the native path (source + declared headers + transitive
deps + defines/flags/incdirs), keyed by the emcc --version line and the
wasm triple so emsdk upgrades and wasm32/64 variants never collide with
each other or with native objects. Cache hits hand the linker the cache
path directly. objectMagicOk accepts the wasm magic. Verified: warm
wasm build of a c-unit drops 1.85s -> 0.61s (emcc -c skipped).
The key previously covered the #source bytes + the block's DECLARED
headers, so a unit whose impl is a thin wrapper over an undeclared
header (vendors/kb_text_shape: two-line impl.c, all code in
kb/kb_text_shape.h) would serve STALE cached objects after an
upstream upgrade. collectIncludeDepBytes now walks the transitive
closure of quoted #include lines (includer-dir first, then -I dirs;
angle/system includes never participate; unresolvable names skip) and
the dep contents fold into the key — no sidecar, no compare logic, a
changed header is just a different key. Verified live: appending to
kb_text_shape.h mints a new cache entry; reverting hits the old one.
Source bytes, declared-header CONTENT (header edits invalidate),
defines/flags/include dirs in order, LLVM version, and target
triple/sysroot all participate; section tags keep equal strings in
different roles distinct. Pure function + variance property tests;
nothing consumes it yet.
