t27c lexer: <<= and >>= are not lexed, so shift compound assignment does not parse #7940

Part of #5980 (P1: compound assignment). The self-hosted core accepts += -= *= /= %= |= &= ^= byte-identically with gen-c (#7313), and refuses <<= and >>= because t27c cannot parse them.

Defect

The emitter maps both operators (bootstrap/src/compiler.rs, the compound-operator table: "<<=" => Some("<<"), ">>=" => Some(">>"), written there so "the next spec to use one must not pay for" a missing entry). But the lexer reads <<= as << followed by =, so a spec that uses either one fails to parse in every backend:

module m;
fn f() -> void {
    var x: i64 = 1;
    x <<= 1;
}

t27c gen-c fails with: parse error in fn 'f' near line 5: Unexpected token in expression: Equals ('=') at line 5:9. The same text is recorded for bootstrap/tests/fixtures/core_selfhost/refuse/e2_shift_left_assign.t27 in tools/specs_generate_baseline.txt.

Done when

The lexer reads <<= and >>= as single tokens and the four backends lower them like the other compound assignments. The core then moves e2_shift_left_assign from refuse/ to accept/, and the line in the specs-generate baseline is removed.

User Scenarios

• Given a T27 source file containing x <<= 1; or x >>= 1;, when t27c gen-c compiles it, then the compilation succeeds (exit 0) and the emitted C code performs the shift compound assignment.
• Given the fixture bootstrap/tests/fixtures/core_selfhost/refuse/e2_shift_left_assign.t27, when the core test suite runs, then the test moves from refuse/ to accept/ and passes.

Requirements

• FR-001: The lexer specification MUST recognize <<= and >>= as single ShiftLeftAssign and ShiftRightAssign tokens (not as ShiftLeft + Equals / ShiftRight + Equals).
• FR-002: All four backends (gen-c, gen-rust, gen-wasm, gen-js) MUST lower ShiftLeftAssign and ShiftRightAssign identically to the existing compound-assignment operators.
• FR-003: The test fixture bootstrap/tests/fixtures/core_selfhost/refuse/e2_shift_left_assign.t27 MUST be moved to bootstrap/tests/fixtures/core_selfhost/accept/ and pass.
• FR-004: The line referencing e2_shift_left_assign.t27 in tools/specs_generate_baseline.txt MUST be removed.

Boundary

• specs/compiler/lexer.t27
• bootstrap/tests/fixtures/core_selfhost/refuse/e2_shift_left_assign.t27
• tools/specs_generate_baseline.txt
• compiler/parser/lexer.t27
• bootstrap/src/compiler.rs