sx: format_number helper — defuse int_of_float overflow on huge floats

Shared formatter in sx_types.ml. Small integer-valued floats still print
as plain ints; floats outside safe-int range (|n| >= 1e16) now print as
%.17g (full precision) instead of silently wrapping to negative or 0.
Non-integer values keep %g 6-digit behavior — no existing SX tests regress.

Unblocks Number.MAX_VALUE / Math.pow(2,N) style tests in js-on-sx where
iterative float loops were collapsing to 0 at ~2^63.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/playwright/generate-sx-tests.py

@@ -1774,8 +1774,9 @@ def extract_hs_expr(raw):
     expr = raw.strip().replace('\n', ' ').replace('\t', ' ')
     # Collapse multiple spaces
     expr = re.sub(r'\s+', ' ', expr)
-    # Escape quotes for SX string
-    expr = expr.replace('\\', '').replace('"', '\\"')
+    # Escape backslashes (preserve regex escapes like \d, CSS escapes, lambda \)
+    # then escape quotes for SX string.
+    expr = expr.replace('\\', '\\\\').replace('"', '\\"')
     return expr