ERR/E4.1b: #caller_location + Source_Location (+ namespaced default fix, comptime flush)

Finishes Phase E4. `process.exit` / `assert` now report the caller's location.

#caller_location + Source_Location:
- new `hash_caller_location` token (lexer) + a leaf `caller_location` AST node
  (parser primary-expr; sema + lsp arms).
- `Source_Location :: struct { file; line; col; func }` in std.sx.
- expandCallDefaults rewrites a `#caller_location` param default to a marker
  carrying the CALL site's span + source_file.
- lowerCallerLocation synthesizes the struct: file + line:col via
  errors.SourceLoc.compute over the diagnostics file→source map, stamped with
  the enclosing (caller) function name. inferExprType resolves it to
  Source_Location. Explicitly forwarding a Source_Location through an inner
  call preserves the outermost site.

namespaced default-param expansion (pre-existing crash): expandCallDefaults
bailed on field_access callees, so `mod.fn(args)` with an omitted defaulted
param passed too few args → LLVM "incorrect number of arguments". Now resolves
the namespace fd (by field / qualified name); method-on-value calls (where
`self` shifts the count) stay excluded. Prerequisite for process.exit/assert
(always called namespaced) taking `loc = #caller_location`.

comptime flush: interp.callForeign flushes the interpreter's buffered print
output before invoking any host symbol, so a comptime diagnostic emitted just
before a terminating `_exit` (process.exit at comptime) survives.

process.exit/assert take `loc: Source_Location = #caller_location`; assert
prints `ASSERTION FAILED at <file>:<line>: <msg>`. examples 250 (assert
file:line), 251 (caller-location + forwarding). The two ffi-objc *.ir
snapshots are regenerated — adding Source_Location to std.sx renumbers the
global string pool the type/field-name tables index (benign, identical IR).

src/sema.zig

@@ -1136,6 +1136,7 @@ pub const Analyzer = struct {
             .try_expr => |te| {
                 try self.analyzeNode(te.operand);
             },
+            .caller_location => {}, // leaf marker (ERR E4.1b) — no sub-nodes
             .catch_expr => |ce| {
                 try self.analyzeNode(ce.operand);
                 try self.pushScope();
@@ -1587,6 +1588,7 @@ pub fn findNodeAtOffset(node: *Node, offset: u32) ?*Node {
             if (findNodeAtOffset(se.operand, offset)) |found| return found;
         },
         .break_expr, .continue_expr => {},
+        .caller_location => {},
         .assignment => |asgn| {
             if (findNodeAtOffset(asgn.target, offset)) |found| return found;
             if (findNodeAtOffset(asgn.value, offset)) |found| return found;