Modular test architecture: per-module test specs for SX

Split monolithic test.sx into composable test specs:
- test-framework.sx: deftest/defsuite macros + assertion helpers
- test-eval.sx: core evaluator + primitives (81 tests)
- test-parser.sx: parser + serializer + round-trips (39 tests)
- test-router.sx: route matching from router.sx (18 tests)
- test-render.sx: HTML adapter rendering (23 tests)

Runners auto-discover specs and test whatever bootstrapped code
is available. Usage: `run.js eval parser router` or just `run.js`.
Legacy mode (`--legacy`) still runs monolithic test.sx.

Router tests use bootstrapped functions (sx_ref.py / sx-browser.js)
because the hand-written evaluator's flat-dict env model doesn't
support set! mutation across lambda closure boundaries.

JS: 161/161. Python: 159/161 (2 parser escape bugs found).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/test.sx

@@ -1,16 +1,21 @@
 ;; ==========================================================================
-;; test.sx — Self-hosting SX test framework
+;; test.sx — Self-hosting SX test suite (backward-compatible entry point)
 ;;
-;; Defines a minimal test framework in SX that tests SX — the language
-;; proves its own correctness. The framework is self-executing: any host
-;; that provides 5 platform functions can evaluate this file directly.
+;; This file includes the test framework and core eval tests inline.
+;; It exists for backward compatibility — runners that load "test.sx"
+;; get the same 81 tests as before.
+;;
+;; For modular testing, runners should instead load:
+;;   1. test-framework.sx  (macros + assertions)
+;;   2. One or more test specs: test-eval.sx, test-parser.sx,
+;;      test-router.sx, test-render.sx, etc.
 ;;
 ;; Platform functions required:
-;;   try-call     (thunk)        → {:ok true} | {:ok false :error "msg"}
-;;   report-pass  (name)         → platform-specific pass output
-;;   report-fail  (name error)   → platform-specific fail output
-;;   push-suite   (name)         → push suite name onto context stack
-;;   pop-suite    ()             → pop suite name from context stack
+;;   try-call     (thunk)        -> {:ok true} | {:ok false :error "msg"}
+;;   report-pass  (name)         -> platform-specific pass output
+;;   report-fail  (name error)   -> platform-specific fail output
+;;   push-suite   (name)         -> push suite name onto context stack
+;;   pop-suite    ()             -> pop suite name from context stack
 ;;
 ;; Usage:
 ;;   ;; Host injects platform functions into env, then: