bugc: carry the call-site source range on the TCO back-edge JUMP

[gnidan]

When tail-call optimization rewrites a tail-recursive call into a
back-edge JUMP, that JUMP carries a flat {return, invoke, transform: ["tailcall"]} context so a debugger still sees the recursive
call in the trace. Until now it mapped only to the callee declaration —
the call expression the user wrote had no instruction pointing back to
it.

This carries the call site's own code range on the back-edge JUMP,
composed flat alongside the invoke/return (disjoint keys). It completes
the call-site source-range story that #263 established for a real caller
JUMP ({invoke, code}) and its continuation JUMPDEST ({return, code}),
and #267 carried through inlining — now the tail-call case matches.

The call expression's range already lives on the original call
terminator's operationDebug; the TCO pass captures it onto the
TailCall IR object (mirroring the existing declarationLoc /
declarationSourceId fields), and codegen composes it into the JUMP's
context. Emission-only: no execution semantics change, and the range is
dropped cleanly when a function has no source location.

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-07-23 22:16 UTC

[gnidan]

Review notes from the format/pointers side:

• Flat composition is correct here — the back-edge JUMP's context is built fresh in buildTailCallJumpOptions ({return, invoke} + code spread flat, transform appended). A runtime dump at level 3 confirms a single flat object with disjoint keys (return, invoke, code, transform), so flat is right and gather would be wrong per the spec's when-to-gather guidance.
• The code shape matches what bugc: carry the call-site source range on invoke/return contexts #263 emits on the real-caller JUMP ({source: {id}, range}), and the emitted range is the recursive call expression itself, correctly distinct from the function's declaration range.
• Compiled the tail-recursion example at levels 2 and 3 and validated the runtime program against schema:ethdebug/format/program — valid at both.
• callSiteLoc/callSiteSourceId mirror the existing declarationLoc/declarationSourceId pattern, and missing call-site info degrades gracefully via guarded spreads. Since bugc never emits compilation, rebuilding the range from the loc pair is lossless in practice.

This completes the call-site story from #263/#267 for the tail-call case.