fix(driver): auto-add the RTL source dir to the loader search path

After the RTL-unification move, stdlib units (classes, contnrs) explicitly
`uses runtime.arc` and friends.  The compiler driver already source-builds the
RTL from compiler/src/main/pascal at link time, but the FRONT-END unit loader
was never told where that source lives — so compiling any program that uses a
stdlib unit (or any RTL unit transitively) failed with
"Unit 'runtime.arc' not found in search paths" unless the user manually passed
--unit-path compiler/src/main/pascal.

Add the RTL source directory to the loader's search paths automatically,
resolved the same way EnsureRTLObjects resolves it for linking: --rtl-src, then
$BLAISE_RTL_SRC, then the binary/CWD-relative RTLSourceDir default.  (stdlib
itself stays an explicit --unit-path, as before — it is an opt-in library, not
implicit like the RTL.)

Regression introduced by the RTL-unification land; reported by a user.
Verified: a `uses classes` program now compiles + runs with only the stdlib
unit-path, from any CWD.  All four fixpoints green; full suite 3831.

compiler/src/main/pascal/Blaise.pas

@@ -31,7 +31,7 @@ uses
   blaise.codegen.native.driver,
   uUnitLoader, uDebugOPDF, uDebugFacts, uUnitInterface, uSemanticExport, uSemanticImport,
   uUnitInterfaceIO, uIfaceObject, uASTDump,
-  blaise.frontend.opts, uConfig;
+  blaise.frontend.opts, uConfig, uToolchain;
 
 type
   { Alias so existing signatures (ParseArgs out param, locals) read
@@ -480,6 +480,7 @@ var
   Front:     TFrontEndOpts;   { front-end-only flag bag, populated by ParseArgs }
   ToolErr:   string;          { CheckToolchain result ('' on success) }
   ValidErr:  string;          { Driver.ValidateOptions result ('' on success) }
+  RTLSrc:    string;          { RTL source dir added to the loader search path }
   PendingFlags: TStringList;  { deferred unknown flags (driver-private) }
   PendingArgs:  TStringList;  { lookahead token per pending flag ('' if none) }
   PendIdx:   Integer;         { drain cursor over the pending lists }
@@ -562,6 +563,24 @@ begin
   if (UnitCacheDir <> '') and (SearchPaths.IndexOf(UnitCacheDir) < 0) then
     SearchPaths.Add(UnitCacheDir);
 
+  { Always make the RTL source directory discoverable to the unit loader.  The
+    RTL units (runtime.*, rtl.platform.*, system) live in the compiler's own
+    source tree after the RTL-unification move; stdlib units such as `classes`
+    explicitly `uses runtime.arc`, so a program that uses them needs the RTL
+    source on the search path — without this the loader fails with
+    "Unit 'runtime.arc' not found".  The driver already source-builds the RTL
+    from this same directory at link time; here we add it for the front-end
+    loader too.  Resolution mirrors EnsureRTLObjects: --rtl-src, then
+    $BLAISE_RTL_SRC, then the binary/CWD-relative default. }
+  RTLSrc := Opts.RTLSrcDir;
+  if RTLSrc = '' then
+    RTLSrc := GetEnvironmentVariable('BLAISE_RTL_SRC');
+  if RTLSrc = '' then
+    RTLSrc := RTLSourceDir();
+  if (RTLSrc <> '') and DirectoryExists(RTLSrc)
+     and (SearchPaths.IndexOf(RTLSrc) < 0) then
+    SearchPaths.Add(RTLSrc);
+
   { Emit-mode / backend compatibility.  --emit-ir prints QBE IR and
     --emit-asm prints native assembly; PickTopDriver routes each to the
     backend that produces it, ignoring --backend.  That silent override is