build: isolate build-info defines into utils/buildinfo.c (ccache-friendly)

MAGIC_VERSION / MAGIC_REVISION / MAGIC_COMMIT / MAGIC_BUILDDATE were on the
global DFLAGS, so every object's compile command line carried them.  Because
the command line is part of ccache/sccache's hash key and MAGIC_BUILDDATE
changes every second, that meant a cache miss on every unit on every build;
it also smeared the baked-in date/commit across "whenever each file last
recompiled".

Move the four values into a single translation unit:
  * utils/magic_buildinfo.h -- extern MagicVersion/MagicRevision/MagicCommit/
    MagicCompileTime (MagicCommit is new; the other three moved here from
    utils/magic.h, which now includes this header).
  * utils/buildinfo.c -- the ONLY unit compiled with the version defines
    (target-specific `buildinfo.o: DFLAGS += ${DFLAGS_MAGICVERSION}` in
    utils/Makefile.in); it defines the four globals.  ${DFLAGS_MAGICVERSION}
    is removed from the global DFLAGS/DFLAGS_NOSTUB.

Consumers updated to read the runtime symbol instead of the compile-time
macro:
  * magicTop.c / tclmagic.c no longer define the globals (the MAGIC_WRAPPER
    duplicate-symbol guard is gone -- buildinfo.o owns them for every variant,
    including WASM, which links both mains).
  * Tcl_PkgProvide/PkgRequire in router/ext2spice/lef/ext2sim/plot/tclmagic
    now pass MagicVersion.
  * extflat: EFVersion (a static-initialized copy of MAGIC_VERSION) is dropped;
    EFread.c compares the .ext version against MagicVersion directly -- the same
    value ExtCell.c already *writes* into .ext files, so read and write are now
    consistent.

Verified: only buildinfo.o carries -DMAGIC_* (hash/DBio/windCmdSZ carry none);
Tcl (375 files) and --without-tcl builds rc=0, version/commit/date embedded in
tclmagic.so, magicTop.o no longer defines MagicVersion; in-tree source clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

ext2spice/ext2spice.c

@@ -206,7 +206,7 @@ Exttospice_Init(
 {
     /* Sanity checks! */
     if (interp == NULL) return TCL_ERROR;
-    if (Tcl_PkgRequire(interp, "Tclmagic", MAGIC_VERSION, 0) == NULL)
+    if (Tcl_PkgRequire(interp, "Tclmagic", MagicVersion, 0) == NULL)
 	return TCL_ERROR;
     if (Tcl_InitStubs(interp, Tclmagic_InitStubsVersion, 0) == NULL) return TCL_ERROR;
 
@@ -223,7 +223,7 @@ Exttospice_Init(
     if (WindReplaceCommand(DBWclientID, "ext2spice", CmdExtToSpice) < 0)
 	return TCL_ERROR;
 
-    Tcl_PkgProvide(interp, "Exttospice", MAGIC_VERSION);
+    Tcl_PkgProvide(interp, "Exttospice", MagicVersion);
     return TCL_OK;
 }
 #endif /* EXT2SPICE_AUTO */