Build: replace fixed SOURCE_DATE_EPOCH fallback

Keep caller-provided SOURCE_DATE_EPOCH authoritative and derive the automatic default through a shared helper used by the Makefile, direct CMake/CPack packaging, and the deb packaging wrapper.

When repository metadata is available, use the HEAD commit timestamp without relying on git -C. Resolve the source root before probing Git so symlinked source paths still use the checkout HEAD. For source tarballs without .git, derive the fallback timestamp from the release date encoded in Common/Tcdefs.h instead of the stale 2020-01-01 constant.

Add TC_RELEASE_DATE_DAY and validate it together with TC_RELEASE_DATE_YEAR, TC_RELEASE_DATE_MONTH, and TC_STR_RELEASE_DATE. Abort when no valid timestamp can be derived.

For direct CMake invocation, initialize SOURCEPATH when the wrapper has not provided it, use the shared helper for derivation, validate the result, and export it for package targets. Also persist the configured epoch through CPACK_PROJECT_CONFIG_FILE so later standalone cpack --config runs export the same value before invoking package generators.

Document that automatic git-checkout builds and release-tarball builds intentionally use different epochs; release reproducers should build from the tarball or set SOURCE_DATE_EPOCH explicitly.

src/Common/Tcdefs.h

@@ -76,6 +76,7 @@ extern unsigned short _rotl16(unsigned short value, unsigned char shift);
 #define TC_STR_RELEASE_DATE			L"June 4, 2026"
 #define TC_RELEASE_DATE_YEAR			2026
 #define TC_RELEASE_DATE_MONTH			 6
+#define TC_RELEASE_DATE_DAY			 4
 
 #define BYTES_PER_KB                    1024LL
 #define BYTES_PER_MB                    1048576LL