self-host: gen-c silently drops every declaration after endmodule #6047

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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

endmodule ends the parse, and anything after it vanishes from the output with exit code 0: the function lost below is never emitted and nothing says so. The self-hosted core refuses a declaration after endmodule (E_SYNTAX). The fix is the same refusal, unless a corpus spec relies on text after endmodule, in which case list them in the PR and stop for a decision.

Reproduce

cargo build --release -p t27c
printf 'module m;\nendmodule\nfn lost() -> i64 {\n    return 1;\n}\n' > /tmp/endm.t27
./target/release/t27c gen-c /tmp/endm.t27

Today the relevant output is:

(header comment only: no prototype, no body for lost, exit 0)

Acceptance criteria

• 1. ./target/release/t27c gen-c /tmp/endm.t27 > /dev/null 2>/tmp/endm.err; echo $? prints a non-zero code (today 0)
• 2. grep -c 'endmodule' /tmp/endm.err prints at least 1: the message names what was refused
• 3. printf 'module m;\nfn kept() -> i64 {\n return 1;\n}\nendmodule\n' > /tmp/endm2.t27 && ./target/release/t27c gen-c /tmp/endm2.t27 | grep -c 'kept(' prints at least 1: endmodule at the end still works
• 4. cargo test --release -p t27c --test genc_after_endmodule passes

Boundary

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

User Scenarios & Testing

• Given /tmp/endm.t27 written by the Reproduce block,
  when ./target/release/t27c gen-c /tmp/endm.t27 runs and its output is compiled with cc,
  then the compiler refuses the file with a message naming endmodule, and a file whose endmodule is last compiles as before.

Requirements

• FR-001: a token after endmodule other than comments and whitespace MUST be refused, never dropped.
• FR-002: list every file under specs/ that has a token after endmodule in the PR (expected: none).
• FR-900: cargo test --release -p t27c --test genc_after_endmodule 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.