A name declared twice in one scope transpiles at exit 0 and gcc rejects the C: two locals in one block, or a body local named like a parameter #2011

Bug

A name declared twice in one scope gets no diagnostic. The transpiler exits 0 and emits both declarations, and gcc rejects the C. Two shapes:

1. Two locals of one name in the same block.
2. A local in a function's outermost block with the name of one of the function's parameters. C gives the parameters and the function's outermost block one scope, so this is a redeclaration too.

ADR-057 allows shadowing "matching C behavior". C forbids a second declaration of a name in the same scope, so neither shape is shadowing.

Found while reviewing PR #1993. That PR changes how the second shape fails (below).

Reproduction (--target host)

u32 total <- 7;

u32 dup() {
    u32 a <- 1;
    u32 a <- 2;
    return a;
}

u32 plain(u32 a) {
    u32 a <- 2;
    return a;
}

u32 shadowing(u32 total) {
    u32 total <- 3;
    return total;
}

On main @ 7d7f27158, exit 0:

uint32_t dup(void) {
    uint32_t a = 1U;
    uint32_t a = 2U;          // gcc: redefinition of 'a'
    ...
uint32_t plain(uint32_t a) {
    uint32_t a = 2U;          // gcc: 'a' redeclared as different kind of symbol
    ...
uint32_t shadowing(uint32_t total) {
    uint32_t shadowing__total = 3U;   // compiles: only the local moved (ADR-057)
    return shadowing__total;
}

On PR #1993 @ 14d9241c9, exit 0. dup and plain are unchanged. shadowing now fails too, because the parameter moves as well (#1290) and the two declarations share one C name:

uint32_t shadowing(uint32_t shadowing__total) {
    uint32_t shadowing__total = 3U;   // gcc: 'shadowing__total' redeclared as different kind of symbol

Control: the same local one block deeper (if (total > 0) { u32 total <- 3; … }) is shadowing. It transpiles on the PR to an inner uint32_t shadowing__total = 3U;, compiles under -Wall -Wextra -Werror, and global.total still reads the global.

Cause

1.4 frames every function-body declaration. Since #1290 that includes the parameters, in the function's frame. Nothing checks a new declaration against the declarations already in its scope, so LexicalFrames.emittedName names both and render emits both.

Definition of done

• A second declaration of a name in one scope is a diagnostic: two locals in the same block, and a local in a function's outermost block named like one of its parameters. One check decides both.
• test-error fixtures for both shapes, including the shadowing-a-global variant of the parameter shape. Beside them goes a control that must stay silent: the same local one block deeper. Each is mutation-checked.