mutual TCO: trampoline tail-edge SCCs (lifts broker's 175-frame cliff)

[samuelSavanovic]

Summary

• Compiler now finds tail-edge SCCs of size ≥ 2 via Tarjan and emits each viable member as a gem_fn_<name>_body plus a thin trampoline-loop wrapper. Intra-SCC tail calls set a global TLB (gem_tail_fn, gem_tail_args, gem_tail_argc, gem_tail_env) and return instead of doing a real C call, so a 7-fn cycle iterates at constant stack depth.
• Direct self-tail recursion still uses the existing while(1)+continue shape inside the body — only cross-fn intra-SCC tail calls take the trampoline.
• Lifts the STOMP broker's 175-frame writer-cycle ceiling (see examples/stomp_broker/NOTES.md "Milestone 6, second pass" for the new sweep numbers — cells previously dead at delivered ≈ N×175 now complete; cells that hit a different cliff survive 3× longer).

What's in the diff

• runtime/gem.h + runtime/gem_error.c: GEM_MAX_TAIL_ARGS = 16, plus the four globals for the TLB.
• compiler/codegen.gem:
  • find_tail_call_sccs (Tarjan + viability filter — bails the whole SCC silently on any member with rest params, defaults, boxed params, name-shadow, or > 16 params).
  • scc_wrapper_for emits the trampoline loop with per-iteration arena reset (gated like emit_tco_continue).
  • emit_scc_tail_call writes the TLB, pops the body's frame, returns GEM_NIL.
  • compile_fn and compile_stmt_return wired to detect SCC membership and intra-SCC tail-call sites.
  • SCC body forward decl added next to the existing wrapper forward decl.
• examples/96_mutual_tco.gem: 2-cycle (even/odd, 20k iters), 3-cycle (broker shape, 50k), and a mixed direct-self-TCO + mutual case (10k). Output appended to expected_output.txt.
• bootstrap/stage0.c regenerated; make bootstrap roundtrip clean on first pass.
• Docs: examples/stomp_broker/NOTES.md second-pass writeup; docs/ROADMAP.md "Deep non-tail recursion ceiling" downgraded P1 → P3 with what's left (catchable stack-overflow, growable stacks); docs/OPTIMIZATIONS_LOG.md shipped entry under Codegen Output; CLAUDE.md Key Decisions updated.

Test plan

• make test — all 124 examples + LSP smokes green.
• make bootstrap — clean roundtrip on first pass; clean rebuild from new stage0.c passes the full suite.
• bash examples/stomp_broker/smoke_test.sh — 4/4 pass.
• bash benchmarks/stomp/sweep.sh — sweep numbers in NOTES.md "Milestone 6, second pass".
• Sanity: examples/96_mutual_tco.gem exercises 2-/3-/5-fn SCCs plus a member with both direct-self TCO and cross-fn cycles in the same body.

Not in this PR

• 1k-fanout characterization (needs GEM_MAX_PROCS=4096 build).
• Slow-consumer to OOM and 100-publisher queue test.
• Backpressure design.

These are the next-tier issues the second-pass cliff (mailbox / process-table) surfaces; per the resume prompt, they're for a later session.