[Backend][Diagnostics] Preserve generated assembly line provenance in internal assembler errors

[a19q3]

Summary

The internal assembler now correctly performs B-type branch relaxation, but most parser, layout, encoding, and machine-CFG failures still construct CompileError with Span::default().

As a result, a malformed or unsupported generated assembly operation can still surface as line 0, even when the assembler knows the input line, operation index, section, label, and byte offset. This was a secondary finding in #1 and remains unresolved after the primary branch-range bug was fixed.

Current evidence

• The assembler parses source lines but stores plain AsmOp values without their originating line number:

  CellScript/src/codegen/assembler.rs

  Lines 340 to 404 in 8ae6dc4

  	fn from_lines_relaxed(lines: &[String], layout: &SectionLayout) -> Result<Self> {
  	let conservative = Self::from_lines_with_branch_mode(lines, BranchSizeMode::Conservative)?;
  	let relaxed_text_branches = conservative.relaxed_branch_indices(layout)?;
  	Self::from_lines_with_branch_mode(lines, BranchSizeMode::Exact(&relaxed_text_branches))
  	}
  	fn from_lines_with_branch_mode(lines: &[String], branch_size_mode: BranchSizeMode<'_>) -> Result<Self> {
  	let mut current_section = SectionKind::Text;
  	let mut text_size = 0usize;
  	let mut rodata_size = 0usize;
  	let mut text_ops = Vec::new();
  	let mut rodata_ops = Vec::new();
  	let mut symbols = HashMap::new();
  	let mut globals = BTreeSet::new();
  	let mut entry_label = None;
  	let mut fallback_entry = None;
  	for line in lines {
  	let Some(clean) = strip_comment(line) else {
  	continue;
  	};
  	if clean.is_empty() {
  	continue;
  	}
  	if let Some(section) = parse_section_directive(clean)? {
  	current_section = section;
  	continue;
  	}
  	if clean.starts_with(".option ") || clean.starts_with(".type ") {
  	continue;
  	}
  	if let Some(symbol) = clean.strip_prefix(".global ") {
  	globals.insert(symbol.trim().to_string());
  	continue;
  	}
  	let (ops, offset) = match current_section {
  	SectionKind::Text => (&mut text_ops, &mut text_size),
  	SectionKind::Rodata => (&mut rodata_ops, &mut rodata_size),
  	};
  	let op_index = ops.len();
  	if let Some(label) = clean.strip_suffix(':') {
  	let label = label.trim().to_string();
  	let symbol = SymbolDef { section: current_section, offset: *offset };
  	if symbols.insert(label.clone(), symbol).is_some() {
  	return Err(CompileError::new(format!("duplicate assembly label '{}'", label), crate::error::Span::default()));
  	}
  	if current_section == SectionKind::Text && globals.contains(&label) {
  	if fallback_entry.is_none() {
  	fallback_entry = Some(label.clone());
  	}
  	if !label.starts_with("__") && entry_label.is_none() {
  	entry_label = Some(label.clone());
  	}
  	}
  	ops.push(AsmOp::Label(label));
  	continue;
  	}
  	let op = parse_asm_op(clean)?;
  	*offset += op_size(&op, *offset, current_section, op_index, branch_size_mode);
  	ops.push(op);
  	}

• Unknown labels, unsupported instructions, invalid immediates, encoding failures, and CFG/layout errors still use Span::default(): https://github.com/CellScript-Labs/CellScript/blob/8ae6dc4a2749fa107ff42c166772600648c19bb7/src/codegen/assembler.rs
• Branch relaxation itself is covered and passing:

  CellScript/src/codegen/assembler.rs

  Lines 2112 to 2131 in 8ae6dc4

  	fn internal_assembler_relaxes_out_of_range_conditional_branch() {
  	let mut lines = vec![
  	".section .text".to_string(),
  	".global entry".to_string(),
  	"entry:".to_string(),
  	"li a0, 0".to_string(),
  	"beqz a0, far_target".to_string(),
  	];
  	for _ in 0..1500 {
  	lines.push("addi t0, t0, 0".to_string());
  	}
  	lines.push("far_target:".to_string());
  	lines.push("ret".to_string());
  	let elf = assemble_elf_internal(&lines).expect("internal assembler should relax long conditional branches");
  	assert!(elf.starts_with(b"\x7fELF"));
  	}
  	#[test]
  	fn internal_assembler_encodes_register_conditional_branches() {

• The complete CellScript test suite is included by the CI gate at

  CellScript/scripts/cellscript_gate.sh

  Lines 580 to 595 in 8ae6dc4

  	require_cmd npm
  	require_node_22
  	printf '{"status":"not-generated","reason":"test suite did not reach backend shape report generation"}\n' >"$CELLSCRIPT_BACKEND_SHAPE_REPORT"
  	cargo_fmt_workspace --check
  	check_canonical_cellscript_format
  	check_example_u64_boundaries
  	run cargo test --locked -p cellscript -- --test-threads=1
  	run cargo test --locked -p cellscript-artifact-checker -- --test-threads=1
  	check_artifact_checker_dependency_boundary
  	run cargo test --locked -p cellscript-fiber-adapter -- --test-threads=1
  	run cargo test --locked -p cellscript-ckb-adapter -- --test-threads=1
  	run cargo test --locked -p cellscript-wasm --features wasm -- --test-threads=1
  	run cargo test --locked -p cellscript-ckb-sdk-builder-example -- --test-threads=1
  	run cargo test --locked -p cellscript-tools -- --test-threads=1
  	run_executable_package_scenarios all

Required diagnostic contract

Preserve at least this provenance for each parsed operation:

• generated assembly line number;
• text/rodata section;
• operation index and byte offset after layout;
• instruction/directive text or a bounded normalized representation;
• target label and resolved displacement for branch/jump failures.

Generated-assembly provenance must not be mislabeled as a CellScript source span. Where codegen has a real source/lowering-map origin, diagnostics may additionally attach it through an explicitly separate field.

All failures caused by user/compiler-produced assembly must remain ordinary stable diagnostics. No parser, layout, or encoding path should panic.

Acceptance criteria

• Parsed assembly operations retain their original generated line number.
• Parse, symbol, layout, branch/jump, immediate, register, directive, and encoding errors report the relevant assembly line.
• Machine-CFG errors identify the involved label/block/operation without pretending to have a CellScript source span.
• Human diagnostics no longer print an unexplained line 0 for an operation with known provenance.
• Machine JSON distinguishes source provenance from generated-assembly provenance.
• Tests cover duplicate/unknown labels, invalid registers/immediates/directives, out-of-range jumps, malformed memory operands, and CFG target errors.
• Existing branch-relaxation and ELF identity tests remain unchanged or are intentionally rebaselined with evidence.
• dev, ci, and backend gates pass.

Non-goals

• reconstructing a CellScript source span when lowering records do not provide one;
• exposing unbounded assembly text in diagnostics;
• changing branch-relaxation semantics;
• replacing the standalone artifact checker's independent diagnostics.