self-host: gen-c writes - -x as --x, a pre-decrement #6046

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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

A unary minus applied to a unary minus is emitted without a space or parentheses, so - -x becomes the C pre-decrement --x. The function then MUTATES its argument and returns x - 1 instead of x. The self-hosted core refuses this shape (E_SYNTAX) rather than copy it.

Reproduce

cargo build --release -p t27c
printf 'module m;\nfn f(x: i64) -> i64 {\n    return - -x;\n}\n' > /tmp/neg.t27
./target/release/t27c gen-c /tmp/neg.t27

Today the relevant output is:

int64_t f(int64_t x) {
    return --x;
}

Acceptance criteria

• 1. ./target/release/t27c gen-c /tmp/neg.t27 | grep -c -- '--x' prints 0 (today 1)
• 2. ./target/release/t27c gen-c /tmp/neg.t27 > /tmp/neg.c && printf '#include "neg.c"\nint main(void){return f(3)==3?0:1;}\n' > /tmp/negm.c && cc -w -o /tmp/negm /tmp/negm.c && /tmp/negm; echo $? prints 0
• 3. cargo test --release -p t27c --test genc_double_negation passes

Boundary

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

User Scenarios & Testing

• Given /tmp/neg.t27 written by the Reproduce block,
  when ./target/release/t27c gen-c /tmp/neg.t27 runs and its output is compiled with cc,
  then f(3) returns 3, and the same holds for - -1, -(-x) and - - -x (which returns -x).

Requirements

• FR-001: a unary operator whose operand starts with the same operator character MUST NOT be glued to it (- -x, - -1, ! !b, ~ ~k).
• FR-900: cargo test --release -p t27c --test genc_double_negation 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.