self-host: gen-c turns a test's write to a global into a new local that shadows it #6052

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You are working as Gamma (Compiler Core) on epic #5980: rewrite t27c in t27 itself.
The self-hosted core (specs/compiler/core/t27core.t27, PR #6041) must write exactly the
bytes t27c gen-c writes, so every gen-c defect is a defect the core has to refuse instead of
learn. Fixing it here is what lets the core grow. Rules for this task:

• Change the Rust emitter only inside the Boundary below. gen/ is generated: never hand-edit it.
• Build with cargo build --release -p t27c and use ./target/release/t27c from the repository
  root, not a t27c on PATH.
• Write the regression test first and watch it fail on the unfixed compiler. Then fix, and
  revert the fix once to watch the test go red again. Say in the PR that you did both.
• Any other spec whose generated C changes must be listed in the PR by path, with one line on
  why the new bytes are right. A silent corpus change is a failed task.
• The PR body says Closes # plus this issue's number and Part of #5980. ASCII only in source files.

Context

Inside a test block, g = 5; where g is a module var is emitted as uint64_t g = 5; (void)g;: a fresh local that shadows the global, so the global never changes and a function reading it sees 0. The test below fails today for that reason. The self-hosted core refuses an assignment to a name not bound in the test (E_TESTASSIGN).

Reproduce

cargo build --release -p t27c
printf 'module m;\nvar g: i64 = 0;\nfn get() -> i64 {\n    return g;\n}\ntest t {\n    g = 5;\n    assert(get() == 5);\n}\n' > /tmp/shadow.t27
./target/release/t27c gen-c /tmp/shadow.t27

Today the relevant output is:

    uint64_t g = 5;
    (void)g;
    assert((get() == 5));   <- fails at run time: get() still sees 0

Acceptance criteria

• 1. ./target/release/t27c gen-c /tmp/shadow.t27 > /tmp/shadow.c && cc -w -DT27_TEST_MAIN -o /tmp/shadowt /tmp/shadow.c && /tmp/shadowt; echo $? prints 0 (today 134, an assertion failure)
• 2. ./target/release/t27c gen-c /tmp/shadow.t27 | grep -c 'uint64_t g = 5' prints 0
• 3. cargo test --release -p t27c --test genc_test_global_write passes

Boundary

• bootstrap/src/compiler.rs
• bootstrap/tests/genc_test_global_write.rs

User Scenarios & Testing

• Given /tmp/shadow.t27 written by the Reproduce block,
  when ./target/release/t27c gen-c /tmp/shadow.t27 runs and its output is compiled with cc,
  then the test passes: the write reaches the module global.

Requirements

• FR-001: an assignment in a test to a name bound at module level MUST write that binding.
• FR-002: an assignment to a name bound nowhere MUST be refused, not declared.
• FR-900: cargo test --release -p t27c --test genc_test_global_write MUST pass, and MUST fail with the fix reverted.
• FR-901: if bootstrap/tests/core_selfhost.rs exists on your base, cargo test --release -p t27c --test core_selfhost MUST still pass.