feat(betlang): wire-first toolchain pass + bet:Dist T on the unified runtime

[hyperpolymath]

What this is

Brings the 2026-06-15 wire-first toolchain pass + bet : Dist T work onto the reconciled main (which already carries the parallel session's TP-5 intro/elim core and the checker's occurs-check + let-polymorphism). The parallel reconciliation merge resolved the TP-5/checker collisions but dropped this session's wiring; this PR re-applies it surgically on top.

Full rationale and the journey's design decisions: docs/decisions/2026-06-15-betlang-wiring-and-bet-dist-t.adoc (8 ADRs).

Highlights

bet : Dist T (option a) — checker + interpreter + codegen aligned with the Lean proof

• check_bet/check_weighted_bet/check_conditional_bet return Dist T; bet { 1,2,3 } + 1 is now correctly rejected. echo(bet …) : Echo (Dist T).
• Interpreter unified onto bet_rt::Value (one runtime): bet → Value::Dist, sample draws; AST lambdas closure-converted into native bet_rt closures. bet-rt and bet-rand are no longer orphaned.
• JS codegen emits distribution objects so sample works end-to-end (verified under deno).

Wire-first pass

• Justfile: was unparseable (// line 1) — fixed + Rust recipes (build-rust/test-rust/check/run/compile/repl-rust).
• Workspace greened: bet-wasm (E0308), bet-viz (im dep + Histogram→Rectangle + textplots lifetimes), bet-chapel (rand_pcg); 3 real bet-rt bugs fixed (queue order, printf %s quoting, msgpack List→Bytes).
• cbindgen no longer strips the SPDX header from generated betlang.h (was an automated licence edit).
• CLI bet compile … --target js|llvm|beam; LSP wired to the real parser + type checker (real parse/type diagnostics with spans; obsolete "LALR conflicts" deps re-enabled).
• Interpreter surfaces unimplemented forms as errors, not silent Unit.
• examples/dice.bet — first working .bet fixture.

Reconciliation choices (honoring owner adjudication)

• TP-5 kept at origin's echoIntro/echoElim core. The richer comonad-law surface (this session's MultiStep shadow of EchoGradedComonad.agda) is TP-5b (deferred, P3) — preserved in local 964d106 + the ADR.
• Origin's occurs-check + let-polymorphism kept (complementary; the audit had flagged both gaps).
• docs/echo-types.adoc left at origin's baseline (owner-managed).

Verification

• 317 workspace tests pass; lake build green; tools/proof-scan.sh clean (no banned escape hatches).
• End-to-end: bet check/run/compile examples/dice.bet all work.

Deferred / for follow-up (ADR-8)

Racket→Rust authority declaration (left as-is); eager-vs-lazy runtime operational model; TP-5b; SEM-1/STAT-1/STAT-2 (Mathlib decision); ABI-1..5 (Idris2 recipe + CI).

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