codegen: make var_addrs misses panic loudly and fix latent struct-field silent drop

Phase 1 of the post-PR-#31 audit. The PR #31 state-local bug had a
specific shape: analyzer allocated a slot, codegen looked it up by
VarId, silently emitted nothing on miss. Six sites in gen_op plus
the global-initializer loop and the parameter-shuffle prologue all
used the same `if let Some(&addr) = self.var_addrs.get(var) { ... }`
pattern with no else branch. Any future allocation-map desync would
slip through the same crack.

Replace every site with a new `IrCodeGen::var_addr(VarId) -> u16`
helper that panics with an explicit "compiler bug" message on miss.
An IR op referencing an unmapped VarId is not valid input — it means
the analyzer and lowerer disagreed on what to allocate, and we want
that crash to surface in CI rather than be absorbed by whatever
zero-filled RAM happened to sit at the read.

Running cargo test against the hardened lookup surfaced exactly the
bug shape the plan predicted: uninitialized struct globals (e.g.
`var p: Point` with no literal initializer) never had their flattened
field VarIds (`"p.x"`, `"p.y"`) registered in var_addrs. The IR
lowerer's `get_or_create_var("p.x")` minted a VarId, the analyzer's
`flatten_struct_fields` allocated an address for it, but IrCodeGen::new
only populated var_addrs from `ir.globals`, which doesn't contain
synthesized field entries for uninitialized structs. Every `p.x = N`
silently compiled to nothing.

Fix by exposing the IR lowerer's name→VarId map on IrProgram and
joining it with the analyzer's allocations in IrCodeGen::new. Every
allocated name that the lowerer knows about now gets a var_addrs
entry. Example ROMs are byte-identical (no example relied on the
dropped writes), but the bug was reachable — any user program with
a plain `var pos: Point` declaration and field writes would have hit
it silently.

Add `uninitialized_struct_field_store_emits_sta_to_allocated_address`
as a byte-level regression guard: compile `p.x = 123` and scan PRG
for `LDA #\$7B / STA <addr>`. Fails against the old silently-dropping
codegen.

https://claude.ai/code/session_01AoQ678uVeqpyayvWHpfDhC

src/optimizer/tests.rs

@@ -22,6 +22,7 @@ fn make_program(ops: Vec<IrOp>, terminator: IrTerminator) -> IrProgram {
         rom_data: vec![],
         states: vec![],
         start_state: String::new(),
+        var_map: std::collections::HashMap::new(),
     }
 }
 
@@ -420,6 +421,7 @@ fn inline_removes_trivial() {
         rom_data: vec![],
         states: vec![],
         start_state: String::new(),
+        var_map: std::collections::HashMap::new(),
     };
 
     inline_small_functions(&mut prog);
@@ -502,6 +504,7 @@ fn const_fold_preserves_loadimm_used_by_sibling_branch() {
         rom_data: vec![],
         states: vec![],
         start_state: String::new(),
+        var_map: std::collections::HashMap::new(),
     };
 
     optimize(&mut prog);